jointcal currently saves its astrometric model as a FITS TAN-SIP, which limits what we can do with the models. Now that SkyWcs is merged, we can just convert the internal model components into ast mappings, put them in a FrameDict, and write that.

This work was actually done on DM-13272, after we discovered errors in the new toSip unittests after DM-10765 was merged. We decided it would be better to just implement what we wanted, than to spend time mucking with the SIP code further.