Implement offline IERS retrieval and cache.

We need LST for Subaru headers. astropy does that "right" and requires UT1, which in turn requires the IERS predictions.

By default these are fetched as needed. Various things can go wrong with getting them, so we should arrange to get them ahead of time and enable the modern astropy mechanism for using that cached file.

This will require updating astropy to 4.0+, which is currently not safe to do since that would update numpy.