Base frames

Base Frames

Base frames are even more abstract than the current FITS frame. They will allow me to not override functions like __call__ in the Analytic Spectral Frame as it is implemented now. This means that combinations of frames will not have to worry about inheritance competition. All calls should then fall through to abstract implementations which throw errors, rather than falling through to bases which can provide functionality.

The advantage of this, is that very abstract calls can uniformly define data structures, but not implement functions which other frames should always override.