Reference Frame Creation

[joaq-ram]

Add pyWWT capability to create and call custom reference frames. This is immediately useful for plotting orbital data. The reference frames for planetary bodies are fixed which means animating orbital data relative to any of these frames will rotate with the planet. This could also be useful for visualizing various planetary features such as magnetic fields around planets using 3D models (model will need to be relative to the planets inertial coordinate frame).

Implementation could look like the following:

• Import custom reference frame and specify an identifier - ex: "Jupiter Inertial"
• Once added can call the custom reference frame in any of the applicable methods (add_table_layer, set_view, solar_system.track_object, etc).