refactor: Fine-grained references support added

src/core/include/units/generic/angle.h

@@ -57,10 +57,16 @@ constexpr auto operator"" _q_rad(long double l) { return angle<radian, long doub
 
 #ifdef UNITS_REFERENCES
 
-namespace references {
+namespace angle_references {
 
 inline constexpr auto rad = reference<dim_angle<>, radian>{};
 
+}  // namespace angle_references
+
+namespace references {
+
+using namespace angle_references;
+
 }  // namespace references
 
 #endif // UNITS_REFERENCES

src/core/include/units/reference.h

@@ -61,12 +61,18 @@ using reference_divide = detail::reference_divide_impl<
  * This type is intended to be used in the quantity references definition:
  *
  * @code{.cpp}
- * namespace references {
+ * namespace length_references {
  * 
  * inline constexpr auto m = reference<dim_length, metre>{};
  * inline constexpr auto km = reference<dim_length, kilometre>{};
  * 
  * }
+ * 
+ * namespace references {
+ * 
+ * using namespace length_references;
+ * 
+ * }
  * @endcode
  * 
  * Quantity references simplify quantity creation: