Generalize CubeFactory classes

[snbianco]

Generalizing the cube factory classes so that TicaCubeFactory is a child of CubeFactory.

A few considerations:

• This is not completely "general" in that the parent CubeFactory class is built for SPOC FFIs. Unfortunately, we can't do much about this without changing the existing API entirely. I did my best to factor all of the TESS-specific content into variables and methods that can be easily overridden in future child classes.
• The tests were kept largely the same, but I did add some assertions and restructured things a bit.
• There was some code in _update_info_table that was opening FITS files within a loop. I fixed this to only open the files once.
• I moved the logic for update_cube into CubeFactory (when it was originally just in TicaCubeFactory) and renamed the method to be public.