Add STANDARD_GRAVITY Constant (#348)

It took me a little while to figure out exactly what I wanted to do
here.

Initially, I was going to _deprecate_ the standard gravity unit.  Then I
realized we need it to define certain other units, such as the
correspondence between "pounds" as a force and a mass --- and I didn't
want to introduce a circular dependency between `:units` and
`:constants`.

Then I was going to deprecate only the _quantity maker_,
`standard_gravity`.  But then I realized it would be weird to create a
situation where certain units have quantity makers, and certain other
ones don't.

In the end, I think the right move is to just provide standard gravity
as a new constant.  This is the preferred way to interact with it, but
the old ways will still be there for the foreseeable future.

Follow-on to #90.

au/code/au/CMakeLists.txt

@@ -51,6 +51,7 @@ header_only_library(
     constants/planck_constant.hh
     constants/reduced_planck_constant.hh
     constants/speed_of_light.hh
+    constants/standard_gravity.hh
     stdx/experimental/is_detected.hh
     stdx/functional.hh
     stdx/type_traits.hh
@@ -239,6 +240,7 @@ gtest_based_test(
     constants/test/planck_constant_test.cc
     constants/test/reduced_planck_constant_test.cc
     constants/test/speed_of_light_test.cc
+    constants/test/standard_gravity_test.cc
   DEPS
     au
     testing

au/code/au/constants/standard_gravity.hh

@@ -0,0 +1,22 @@
+// 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/standard_gravity.hh"
+
+namespace au {
+
+constexpr auto STANDARD_GRAVITY = make_constant(StandardGravity{});
+
+}  // namespace au

au/code/au/constants/test/standard_gravity_test.cc

@@ -0,0 +1,44 @@
+// 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/standard_gravity.hh"
+
+#include "au/testing.hh"
+#include "au/units/meters.hh"
+#include "au/units/seconds.hh"
+#include "gtest/gtest.h"
+
+namespace au {
+namespace {
+
+using ::testing::StrEq;
+
+TEST(StandardGravity, HasExpectedValue) {
+    // g_0 = 9.80665 m/s^2
+
+    // Test approximate value (guard against powers-of-10 type errors).
+    constexpr auto defining_units = meters / pow<2>(seconds);
+    constexpr auto val = defining_units(9.80665);
+    constexpr auto err = defining_units(0.00001);
+    EXPECT_THAT(STANDARD_GRAVITY.as<double>(defining_units), IsNear(val, err));
+
+    // Test exact value.
+    EXPECT_THAT(STANDARD_GRAVITY.in<int>(defining_units / pow<5>(mag<10>())),
+                SameTypeAndValue(980'665));
+}
+
+TEST(StandardGravity, HasExpectedLabel) { EXPECT_THAT(unit_label(STANDARD_GRAVITY), StrEq("g_0")); }
+
+}  // namespace
+}  // namespace au

docs/reference/constant.md

@@ -53,6 +53,7 @@ Here are the constants that we include with Au:
 | Planck constant | $h$ | $6.626\,070\,15 \times 10^{-34}\,\, \text{J} \cdot \text{s}$ | `planck_constant.hh` | `PLANCK_CONSTANT` |
 | Reduced Planck constant | $\hbar$ | $1.054\,571\,817 \times 10^{-34}\,\, \text{J} \cdot \text{s}$ | `reduced_planck_constant.hh` | `REDUCED_PLANCK_CONSTANT` |
 | Speed of light | $c$ | $299\,792\,458\,\, \text{m} / \text{s}$ | `speed_of_light.hh` | `SPEED_OF_LIGHT` |
+| Standard Gravity | $g_0$ | $9.806\,65\,\, \text{m} / \text{s}^2$ | `standard_gravity.hh` | `STANDARD_GRAVITY` |
 
 Our policy is to include only exactly defined constants with the library.  This rules out many
 useful constants, such as the universal gravitational constant $G$, the _new_ (post-2019) permeability