IGRINS cleanup

[phirstgemini]

The DRAGONS igrins adclass.py is a mess.
The whole PHU vs 1st extension thing appears to be because the existing S files have a completely empty (ie not even an INSTRUME header) PHU and everything is in the extension. Or are they single ext FITS and this is what astrodata makes of them?

Not clear what headers have changed over time, this will need some trial and error to clean up. Now that there's no more data, it might be better to fix the data files rather than kludge it in the code.

At this point, there's basically nothing in the IGRINS-2 team's adclass.py. Not obvious if they should use the same class, or if one should be a subclass of the other. Perhaps IGRINS should be a subclass of IGRINS-2, so that when they actually handle actual instrument stuff in the -2 one then the old one will get it, and we can handle the quirks of the visitor instrument file format in the subclass.