Ability to read INPOP ephemerides

[dlakaplan]

It would be helpful to read the European INPOP ephemerides like INPOP21a: https://www.imcce.fr/recherche/equipes/asd/inpop/download21a They claim to make files in the JPL format, but as far as I can tell from a quick look they are not actually compatible. They have different starting data (INPOP21a vs. the DAF/ or NAIF/DAF that jplephem wants) and different structures too.

I don't know how complicated the differences are. I realize that INPOP does provide their own python routines, but we are using astropy which implicitly assumes that the ephemerides will be handled by jplephem, so changes here may be the simplest.