Return the native speed measurement required to achieve desired degrees-per-minute

def to_native_units(self, motor):
        """
        Return the native speed measurement required to achieve desired degrees-per-minute
        """
        assert abs(self.degrees_per_minute) <= motor.max_dpm,\
            "invalid degrees-per-minute: {} max DPM is {}, {} was requested".format(
            motor, motor.max_dpm, self.degrees_per_minute)
        return self.degrees_per_minute/motor.max_dpm * motor.max_speed