Fixes for bounding box units

[WilliamJamieson]

This PR attempts to fix some issues related to having bounding boxes with units in GWCS. In particular it fixes the WCS.invert failing in this case and WCS.to_fits.

Resolves #408.

CC @pllim, @bmorris3

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.96%. Comparing base (877716d) to head (f391f14).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #554      +/-   ##
==========================================
+ Coverage   85.41%   85.96%   +0.54%     
==========================================
  Files          22       27       +5     
  Lines        3957     4025      +68     
==========================================
+ Hits         3380     3460      +80     
+ Misses        577      565      -12     

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[WilliamJamieson]

This is a bit of a hack job, especially in the to_fits. However, I don't think the to_fits code was ever designed to work with units to begin with, so I was just patching things so that when units are required or not the appropriate casting took place

[pllim]

I have no recollection of the issue I opened 3 years ago. 😆

But I did test this branch downstream at spacetelescope/jdaviz#3417 and it was satisfactory. Thanks!