Adds instance cache to expensive operation

[pet1330]

No description provided.

[RaymondCM]

This looks good so far, the only thing I would suggest is that we make def __dict_to_ros_msg_dict(data_dict, ): and def __ros_msg_dict_to_list(ros_msg_dict, return_keys=False, parent=""): class functions and also cache these because they are called by .msgs.