Attempt to satisfy MSVC 14

```
test\unit_test\runtime\magnitude_test.cpp(143): error C2672:
'units::check_same_type_and_value': no matching overloaded function
found
```

Maybe it's confused by accessing the static member variable template
using dot-notation on an _instance_?  If so, let's see how it likes the
member-function syntax.

src/core/include/units/magnitude.h

@@ -339,6 +339,11 @@ struct magnitude {
     requires (std::is_floating_point_v<T> || (detail::is_integral(BPs) && ...))
   static constexpr T value = detail::checked_static_cast<T>(
       (detail::compute_base_power<T>(BPs) * ...));
+
+  // Trying to satisfy MSVC 14...
+  template<typename T>
+    requires (std::is_floating_point_v<T> || (detail::is_integral(BPs) && ...))
+  constexpr T get_value() const { return magnitude::value<T>; }
 };
 
 // Implementation for Magnitude concept (below).