Made quantity_cast more versatile

- One can now change the unit type without changing the representation type.
- One can now change the representation type without changing the unit type.
- Had to implement std::common_type<my_value, my_value> to still compile
  integration tests. (It implicitly converted to another representation
  before.)

test/unit_test/test_quantity.cpp

@@ -62,6 +62,10 @@ namespace units {
 
 namespace std {
 
+  template<typename T, typename U>
+  struct common_type<my_value<T>, my_value<U>> : common_type<T, U> {
+  };
+
   template<typename T, typename U>
   struct common_type<my_value<T>, U> : common_type<T, U> {
   };