NTTP ratio support added (resolves #49)

src/include/units/derived_dimension.h

@@ -82,7 +82,7 @@ template<typename Child, Unit U, Exponent E, Exponent... ERest>
 struct derived_dimension : downcast_child<Child, typename detail::make_dimension<E, ERest...>> {
   using recipe = exp_list<E, ERest...>;
   using coherent_unit = U;
-  using base_units_ratio = detail::base_units_ratio<derived_dimension>;
+  static constexpr ratio base_units_ratio = detail::base_units_ratio(typename derived_dimension::exponents());
 };
 
 }  // namespace units