Base: make UnitsApi::toString method of Quantity

This static method takes Quantity as a parameter, so make it
method of Quantity directly.

src/Base/UnitsApi.cpp

@@ -27,7 +27,6 @@
 #endif
 
 #include <CXX/WrapPython.h>
-#include <fmt/format.h>
 
 #include "Exception.h"
 #include "UnitsApi.h"
@@ -94,11 +93,6 @@ void UnitsApi::setSchema(const size_t num)
     schemas->select(num);
 }
 
-std::string UnitsApi::toString(const Quantity& quantity, const QuantityFormat& format)
-{
-    return fmt::format("'{} {}'", toNumber(quantity, format), quantity.getUnit().getString());
-}
-
 std::string UnitsApi::toNumber(const Quantity& quantity, const QuantityFormat& format)
 {
     return toNumber(quantity.getValue(), format);