Add remaining exact SI constants (#336)

All values were taken from [this section] of the wikipedia page on the
2019 revision of the SI.

Admittedly, two of them --- `Delta_nu_Cs` and `K_cd` --- have horribly
awkward names.  The best I could do for the constant names was
`CESIUM_HYPERFINE_TRANSITION_FREQUENCY` and
`LUMINOUS_EFFICACY_540_TERAHERTZ`, respectively.  That's fine; we expect
most users to do something like this in their programs:

```cpp
constexpr auto K_cd = au::LUMINOUS_EFFICACY_540_TERAHERTZ;
```

Helps #90.

[this section]:
https://en.wikipedia.org/wiki/2019_revision_of_the_SI#Defining_constants

au/code/au/CMakeLists.txt

@@ -43,6 +43,13 @@ header_only_library(
     unit_symbol.hh
     wrapper_operations.hh
     zero.hh
+    constants/avogadro_constant.hh
+    constants/boltzmann_constant.hh
+    constants/cesium_hyperfine_transition_frequency.hh
+    constants/elementary_charge.hh
+    constants/luminous_efficacy_540_terahertz.hh
+    constants/planck_constant.hh
+    constants/reduced_planck_constant.hh
     constants/speed_of_light.hh
     stdx/experimental/is_detected.hh
     stdx/functional.hh
@@ -224,6 +231,13 @@ gtest_based_test(
 gtest_based_test(
   NAME constants_test
   SRCS
+    constants/test/avogadro_constant_test.cc
+    constants/test/boltzmann_constant_test.cc
+    constants/test/cesium_hyperfine_transition_frequency_test.cc
+    constants/test/elementary_charge_test.cc
+    constants/test/luminous_efficacy_540_terahertz_test.cc
+    constants/test/planck_constant_test.cc
+    constants/test/reduced_planck_constant_test.cc
     constants/test/speed_of_light_test.cc
   DEPS
     au

au/code/au/constants/avogadro_constant.hh

@@ -0,0 +1,37 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constant.hh"
+#include "au/units/moles.hh"
+
+namespace au {
+
+namespace detail {
+// DO NOT follow this pattern to define your own units.  This is for library-defined units.
+// Instead, follow instructions at (https://aurora-opensource.github.io/au/main/howto/new-units/).
+template <typename T>
+struct AvogadroConstantLabel {
+    static constexpr const char label[] = "N_A";
+};
+template <typename T>
+constexpr const char AvogadroConstantLabel<T>::label[];
+struct AvogadroConstantUnit : decltype(inverse(Moles{}) * mag<602'214'076>() * pow<15>(mag<10>())),
+                              AvogadroConstantLabel<void> {
+    using AvogadroConstantLabel<void>::label;
+};
+}  // namespace detail
+
+constexpr auto AVOGADRO_CONSTANT = make_constant(detail::AvogadroConstantUnit{});
+
+}  // namespace au

au/code/au/constants/boltzmann_constant.hh

@@ -0,0 +1,39 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constant.hh"
+#include "au/units/joules.hh"
+#include "au/units/kelvins.hh"
+
+namespace au {
+
+namespace detail {
+// DO NOT follow this pattern to define your own units.  This is for library-defined units.
+// Instead, follow instructions at (https://aurora-opensource.github.io/au/main/howto/new-units/).
+template <typename T>
+struct BoltzmannConstantLabel {
+    static constexpr const char label[] = "k_B";
+};
+template <typename T>
+constexpr const char BoltzmannConstantLabel<T>::label[];
+struct BoltzmannConstantUnit
+    : decltype((Joules{} / Kelvins{}) * mag<1'380'649>() * pow<-29>(mag<10>())),
+      BoltzmannConstantLabel<void> {
+    using BoltzmannConstantLabel<void>::label;
+};
+}  // namespace detail
+
+constexpr auto BOLTZMANN_CONSTANT = make_constant(detail::BoltzmannConstantUnit{});
+
+}  // namespace au

au/code/au/constants/cesium_hyperfine_transition_frequency.hh

@@ -0,0 +1,38 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constant.hh"
+#include "au/units/hertz.hh"
+
+namespace au {
+
+namespace detail {
+// DO NOT follow this pattern to define your own units.  This is for library-defined units.
+// Instead, follow instructions at (https://aurora-opensource.github.io/au/main/howto/new-units/).
+template <typename T>
+struct CesiumHyperfineTransitionFrequencyLabel {
+    static constexpr const char label[] = "Delta_nu_Cs";
+};
+template <typename T>
+constexpr const char CesiumHyperfineTransitionFrequencyLabel<T>::label[];
+struct CesiumHyperfineTransitionFrequencyUnit : decltype(Hertz{} * mag<9'192'631'770>()),
+                                                CesiumHyperfineTransitionFrequencyLabel<void> {
+    using CesiumHyperfineTransitionFrequencyLabel<void>::label;
+};
+}  // namespace detail
+
+constexpr auto CESIUM_HYPERFINE_TRANSITION_FREQUENCY =
+    make_constant(detail::CesiumHyperfineTransitionFrequencyUnit{});
+
+}  // namespace au

au/code/au/constants/elementary_charge.hh

@@ -0,0 +1,37 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constant.hh"
+#include "au/units/coulombs.hh"
+
+namespace au {
+
+namespace detail {
+// DO NOT follow this pattern to define your own units.  This is for library-defined units.
+// Instead, follow instructions at (https://aurora-opensource.github.io/au/main/howto/new-units/).
+template <typename T>
+struct ElementaryChargeLabel {
+    static constexpr const char label[] = "e";
+};
+template <typename T>
+constexpr const char ElementaryChargeLabel<T>::label[];
+struct ElementaryChargeUnit : decltype(Coulombs{} * mag<1'602'176'634>() * pow<-28>(mag<10>())),
+                              ElementaryChargeLabel<void> {
+    using ElementaryChargeLabel<void>::label;
+};
+}  // namespace detail
+
+constexpr auto ELEMENTARY_CHARGE = make_constant(detail::ElementaryChargeUnit{});
+
+}  // namespace au

au/code/au/constants/luminous_efficacy_540_terahertz.hh

@@ -0,0 +1,39 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constant.hh"
+#include "au/units/lumens.hh"
+#include "au/units/watts.hh"
+
+namespace au {
+
+namespace detail {
+// DO NOT follow this pattern to define your own units.  This is for library-defined units.
+// Instead, follow instructions at (https://aurora-opensource.github.io/au/main/howto/new-units/).
+template <typename T>
+struct LuminousEfficacy540TerahertzLabel {
+    static constexpr const char label[] = "K_cd";
+};
+template <typename T>
+constexpr const char LuminousEfficacy540TerahertzLabel<T>::label[];
+struct LuminousEfficacy540TerahertzUnit : decltype((Lumens{} / Watts{}) * mag<683>()),
+                                          LuminousEfficacy540TerahertzLabel<void> {
+    using LuminousEfficacy540TerahertzLabel<void>::label;
+};
+}  // namespace detail
+
+constexpr auto LUMINOUS_EFFICACY_540_TERAHERTZ =
+    make_constant(detail::LuminousEfficacy540TerahertzUnit{});
+
+}  // namespace au

au/code/au/constants/planck_constant.hh

@@ -0,0 +1,39 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constant.hh"
+#include "au/units/joules.hh"
+#include "au/units/seconds.hh"
+
+namespace au {
+
+namespace detail {
+// DO NOT follow this pattern to define your own units.  This is for library-defined units.
+// Instead, follow instructions at (https://aurora-opensource.github.io/au/main/howto/new-units/).
+template <typename T>
+struct PlanckConstantLabel {
+    static constexpr const char label[] = "h";
+};
+template <typename T>
+constexpr const char PlanckConstantLabel<T>::label[];
+struct PlanckConstantUnit
+    : decltype(Joules{} * Seconds{} * mag<662'607'015>() * pow<-42>(mag<10>())),
+      PlanckConstantLabel<void> {
+    using PlanckConstantLabel<void>::label;
+};
+}  // namespace detail
+
+constexpr auto PLANCK_CONSTANT = make_constant(detail::PlanckConstantUnit{});
+
+}  // namespace au

au/code/au/constants/reduced_planck_constant.hh

@@ -0,0 +1,39 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constant.hh"
+#include "au/units/joules.hh"
+#include "au/units/seconds.hh"
+
+namespace au {
+
+namespace detail {
+// DO NOT follow this pattern to define your own units.  This is for library-defined units.
+// Instead, follow instructions at (https://aurora-opensource.github.io/au/main/howto/new-units/).
+template <typename T>
+struct ReducedPlanckConstantLabel {
+    static constexpr const char label[] = "h_bar";
+};
+template <typename T>
+constexpr const char ReducedPlanckConstantLabel<T>::label[];
+struct ReducedPlanckConstantUnit : decltype(Joules{} * Seconds{} * mag<662'607'015>() *
+                                            pow<-42>(mag<10>()) / mag<2>() / Magnitude<Pi>{}),
+                                   ReducedPlanckConstantLabel<void> {
+    using ReducedPlanckConstantLabel<void>::label;
+};
+}  // namespace detail
+
+constexpr auto REDUCED_PLANCK_CONSTANT = make_constant(detail::ReducedPlanckConstantUnit{});
+
+}  // namespace au

au/code/au/constants/test/avogadro_constant_test.cc

@@ -0,0 +1,45 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constants/avogadro_constant.hh"
+
+#include "au/testing.hh"
+#include "au/units/moles.hh"
+#include "gtest/gtest.h"
+
+namespace au {
+namespace {
+
+using ::testing::StrEq;
+
+TEST(AvogadroConstant, HasExpectedValue) {
+    // N_A = 6.02214076e23 mol^(-1)
+
+    // Test approximate value (guard against powers-of-10 type errors).
+    constexpr auto defining_units = inverse(moles) * pow<23>(mag<10>());
+    constexpr auto val = defining_units(6.02214076);
+    constexpr auto err = defining_units(0.00000001);
+    EXPECT_THAT(AVOGADRO_CONSTANT.as<double>(inverse(moles)), IsNear(val, err));
+
+    // Test exact value.
+    EXPECT_THAT(AVOGADRO_CONSTANT.in<int>(defining_units / pow<8>(mag<10>())),
+                SameTypeAndValue(602'214'076));
+}
+
+TEST(AvogadroConstant, HasExpectedLabel) {
+    EXPECT_THAT(unit_label(AVOGADRO_CONSTANT), StrEq("N_A"));
+}
+
+}  // namespace
+}  // namespace au

au/code/au/constants/test/boltzmann_constant_test.cc

@@ -0,0 +1,48 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constants/boltzmann_constant.hh"
+
+#include "au/testing.hh"
+#include "au/units/joules.hh"
+#include "au/units/kelvins.hh"
+#include "gtest/gtest.h"
+
+namespace au {
+namespace {
+
+using symbols::J;
+using symbols::K;
+using ::testing::StrEq;
+
+TEST(BoltzmannConstant, HasExpectedValue) {
+    // k_B = 1.380649e-23 J/K
+
+    // Test approximate value (guard against powers-of-10 type errors).
+    constexpr auto defining_units = (joules / kelvin) * pow<-23>(mag<10>());
+    constexpr auto val = defining_units(1.380649);
+    constexpr auto err = defining_units(0.000001);
+    EXPECT_THAT(BOLTZMANN_CONSTANT.as<double>(J / K), IsNear(val, err));
+
+    // Test exact value.
+    EXPECT_THAT(BOLTZMANN_CONSTANT.in<int>(defining_units / pow<6>(mag<10>())),
+                SameTypeAndValue(1'380'649));
+}
+
+TEST(BoltzmannConstant, HasExpectedLabel) {
+    EXPECT_THAT(unit_label(BOLTZMANN_CONSTANT), StrEq("k_B"));
+}
+
+}  // namespace
+}  // namespace au