Refactor coordinates API to use APE 14

[astrofrog]

This PR changes the Coordinates infrastructure to now operate on coordinate objects compliant with the universal APE 14 WCS API developed by the Astropy project (but not specific to astronomy). This will make it easier to make use of e.g. GWCS, and also means that we no longer need to rely on the hack of trying to translate various coordinate classes to WCS objects for the purposes of display in WCSAxes (which can work with any APE 14-compatible coordinates). We can also then leverage any utilities developed for this API for free without having to write our own versions.

This is not yet finished - in particular, I need to:

• Try and remove the pixel2world and world2pixel methods from the various classes
• Re-work AffineCoordinates to no longer use the WCS class
• Update the documentation
• Check for untested uses of the coordinates API
• Check the impact for other plugin packages
• Write a 'What's new' entry to explain the changes in case it impacts users

Fixes #2061

[codecov]

Codecov Report

Merging #2079 into master will decrease coverage by 2.31%.
The diff coverage is 90.83%.

@@            Coverage Diff             @@
##           master    #2079      +/-   ##
==========================================
- Coverage   90.27%   87.95%   -2.32%     
==========================================
  Files         394      243     -151     
  Lines       39190    22090   -17100     
==========================================
- Hits        35377    19430   -15947     
+ Misses       3813     2660    -1153

Impacted Files	Coverage Δ
glue/_deps.py	66.38% <ø> (+7.93%)	⬆️
glue/core/data_factories/fits.py	88.88% <ø> (+1.29%)	⬆️
glue/plugins/wcs_autolinking/__init__.py	100% <ø> (+14.28%)	⬆️
glue/plugins/coordinate_helpers/link_helpers.py	100% <ø> (ø)	⬆️
glue/viewers/image/qt/standalone_image_viewer.py	94.89% <ø> (-0.35%)	⬇️
glue/core/component.py	97% <100%> (+0.5%)	⬆️
glue/core/data_exporters/gridded_fits.py	90.69% <100%> (ø)	⬆️
glue/core/component_link.py	86.48% <100%> (+0.05%)	⬆️
glue/viewers/profile/state.py	98.82% <100%> (ø)	⬆️
glue/core/state.py	92.06% <100%> (+0.36%)	⬆️
... and 188 more

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