Add on spring cleaning

[nden]

This PR adds on #457. It includes all changes by @Cadair in that PR. All, but the SlicedWCS, tests pass locally.

@Cadair @perrygreenfield

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.96774% with 10 lines in your changes missing coverage. Please review.

Project coverage is 87.05%. Comparing base (8117b3c) to head (4685bcd). Report is 20 commits behind head on master.

Files with missing lines	Patch %	Lines
gwcs/coordinate_frames.py	97.05%	5 Missing :warning:
gwcs/wcs.py	91.37%	5 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #486      +/-   ##
==========================================
- Coverage   87.25%   87.05%   -0.20%     
==========================================
  Files          22       22              
  Lines        3814     3717      -97     
==========================================
- Hits         3328     3236      -92     
+ Misses        486      481       -5     

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[Cadair]

I just discovered another bug that's fixed by this, so I guess it's time for me to resurrect it again

[Cadair]

I have merged this back into #457 so I can keep pushing there. This can be closed.