Combined ephem functions

[hannorein]

1. This removes the wrapper functions ast_ephem and planet_ephem from forces.c. These really don't do much other than look up the body's mass in an array and call the corresponding function assist_jpl_calc and assist_spk_calc. Now that little extra functionality is just all in assist_jpl_calc and assist_spk_calc which I would think makes it easier to maintain. (This was originally also part of the big PR that we've put on hold for now)
2. This provides an ASSIST-wide enum ASSIST_STATUS. Previously a similar enum was declared in multiple places which could be problematic if they ever get out of sync.