Refactor Coordinate Frames

[WilliamJamieson]

This PR cleans up the coordinate_frames module, splitting it into submodules and fully implementing type hints to describe its API.

Note that this PR is built on #562 and #563 so those PRs should be merged first.

[codecov]

Codecov Report

❌ Patch coverage is 93.47368% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.16%. Comparing base (f7128a3) to head (7af634b).
⚠️ Report is 69 commits behind head on master.

Files with missing lines	Patch %	Lines
gwcs/coordinate_frames/_empty.py	72.00%	14 Missing ⚠️
gwcs/coordinate_frames/_core.py	90.80%	8 Missing ⚠️
gwcs/coordinate_frames/_composite.py	93.24%	5 Missing ⚠️
gwcs/coordinate_frames/_properties.py	92.45%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #567      +/-   ##
==========================================
+ Coverage   86.03%   86.16%   +0.13%     
==========================================
  Files          27       38      +11     
  Lines        4045     4092      +47     
==========================================
+ Hits         3480     3526      +46     
- Misses        565      566       +1     

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[WilliamJamieson]

Should this even be in coordinate_frames? It does not appear to be referenced anywhere inside it. It seems to actually be used only by the WCS so maybe move it into wcs._utils?

[mcara]

@nden could answer this better, but it seems that while it is not directly related to coordinate frames, ucd/ctype is important for axes types that are, in turn, related to the coordinate frames. However, it does not seem to me that the location of this is of much importance, especially if private.

[WilliamJamieson]

@nden, do you have a preference?

[nden]

I don't. Perhaps in the future there will be more need for coordinate_frames/utils but I can't think of anything right now. I'm fine if you want to move it.

[Cadair]

I really should finish #461

[perrygreenfield]

Only a minor comment.

[nden]

LGTM. Can you run the regression tests for both pipelines?

[WilliamJamieson]

The regression tests indeed pass.