BaseFrame Property Setter

@preferred_representation.setter
    def preferred_representation(self, value):
        # In reality do validation of `value` and allow string input
	rep_dict = {'spherical': SphericalRepresentation,
		    'cartesian': CartesianRepresentation,
                    'unitspherical': UnitSphericalRepresentation,
                    'cylindrical': CylindricalRepresentation}

        # If value is a string, parse it
        if type(value) is str and value.lower() in rep_dict:
	    value = rep_dict[value.lower()]
        # Converted to representation object, now check allowed representations
        if value in self._representations:
            self._preferred_representation = value
        else:
            raise ValueError('{0} is not a valid representation for {1}'.format(value, self.__class__))