Support QuantityPoint in isnan (#236)

Helps #221.

au/math.hh

@@ -309,6 +309,12 @@ constexpr bool isnan(Quantity<U, R> q) {
     return std::isnan(q.in(U{}));
 }
 
+// Overload of `isnan` for `QuantityPoint`.
+template <typename U, typename R>
+constexpr bool isnan(QuantityPoint<U, R> p) {
+    return std::isnan(p.in(U{}));
+}
+
 // The maximum of two values of the same dimension.
 //
 // Unlike std::max, returns by value rather than by reference, because the types might differ.

au/math_test.cc

@@ -601,6 +601,7 @@ TEST(isnan, TransparentlyActsOnSameAsValue) {
 
     for (const double x : values) {
         EXPECT_EQ(isnan(meters(x)), std::isnan(x));
+        EXPECT_EQ(isnan(meters_pt(x)), std::isnan(x));
         EXPECT_EQ(isnan((radians / second)(x)), std::isnan(x));
     }
 }