Initial fix for issue 2555
Small modifications to enable keyword retrieval
Pass requested keywords as a string.
Allow multiple --key arguments
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
--keyword instead of --key
--keyword instead of --key
Warn on invalid WCS unit
CHANGES entry
update setup_helpers and a corresponding setup.py change
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