Load a value, converting it to the proper type if validation_type exists.

def load(self, value):
        """Load a value, converting it to the proper type if validation_type exists."""
        if self.property_type is None:
            return value
        elif not isinstance(self.property_type, BaseType):
            raise TypeError('property_type must be schematics BaseType')
        else:
            native_value = self.property_type.to_native(value)
            self.property_type.validate(native_value)
            return native_value