CCDData doesn't support non-FITS WCS

[astrofrog]

Currently CCDData seems to only accept FITS WCS:

astropy/astropy/nddata/ccddata.py

Lines 191 to 192 in f5cada0

	if not isinstance(llwcs, WCS):
	raise TypeError("the wcs must be a WCS instance.")

This seems overly restrictive, and it would be better to support any APE 14 WCS? If some specific methods don't support non-FITS WCS it might be better to disable those specific methods/features rather than not allow CCDData to be instantiated?

[astrofrog]

This came up in spacetelescope/gwcs#362

[astrofrog]

For instance it would be reasonable for to_hdu to raise an error if the WCS is not FITS WCS since it is a FITS-specific feature.