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.)

src/include/units/quantity.h

@@ -135,11 +135,21 @@ namespace units {
     return cast::cast(q);
   }
 
-  template<Unit ToU, Scalar ToRep = double, typename U, typename Rep>
+  template<Unit ToU, Scalar ToRep, typename U, typename Rep>
   constexpr quantity<ToU, ToRep> quantity_cast(const quantity<U, Rep>& q)
   {
     return quantity_cast<quantity<ToU, ToRep>>(q);
   }
+  template<Unit ToU, typename U, typename Rep>
+  constexpr quantity<ToU, Rep> quantity_cast(const quantity<U, Rep>& q)
+  {
+    return quantity_cast<quantity<ToU, Rep>>(q);
+  }
+  template<Scalar ToRep, typename U, typename Rep>
+  constexpr quantity<U, ToRep> quantity_cast(const quantity<U, Rep>& q)
+  {
+    return quantity_cast<quantity<U, ToRep>>(q);
+  }
 
 
   // quantity_values

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> {
   };