Currently, only TAN and TAN-SIP WCSs can be persisted to the Wcs extension when we write Exposures to FITS files. We need to extend this to support at least the commonly-encountered WCS projections.

I'm using WCS here to mean Calabretta-and-Greisen WCSs, as in general our Wcs objects may not map onto the FITS standard. We will need to be able to persist more general (and probably composed) maps to disk.