Make WCS instances use WCSAxes if available
Added simple documentation
Added changelog entry [ci skip]
Added URL for more details on WCSAxes
Make comment into docstring as suggested by @mhvk
Warn on invalid WCS unit
CHANGES entry
Upgrade to wcslib 4.25
update setup_helpers and a corresponding setup.py change
Merge pull request #3204 from mdboom/wcs/upgrade-4_25

Upgrade to wcslib 4.25
skip doctest with matplotlib in skycoord for stability
Clarify what is returned
Add mad_std function
Add fwhm_to_sigma and sigma_to_fwhm functions
Implement @embray's comments
Merge pull request #3183 from astrofrog/wcsaxes-compatibility

WCSAxes compatibility hook
Prepend "gaussian_" to FWHM <-> sigma conversion functions
Added instructions on setting up affiliated package manually
Added section on updating for manual set-up
Added instructions on updating astropy-helpers