parse a scene.velocity message

def scene_velocity(frames):
        """parse a scene.velocity message"""
        # "scene.velocity" <scene_id> <velocity>
        reader = MessageReader(frames)
        results = reader.string("command").uint32("scene_id").uint32("velocity").assert_end().get()
        if results.command != "scene.velocity":
            raise MessageParserError("Command is not 'scene.velocity'")
        return (results.scene_id, results.velocity/1000)