Add GWCS support to calc_pixmap()

[jdavies-st]

trafficstars

Currently an implementation of this is in the jwst package.

https://github.com/spacetelescope/jwst/blob/bbe04525a7ced5354ed5457798457215bb33d29c/jwst/resample/resample_utils.py#L98-L162

Update both the FITS WCS and GWCS code to use the APE 14 enhancements where possible.