Fixed find_optimal_celestial_wcs for cases where SkyCoords can't be stacked

[astrofrog]

Possible fix for #583 - fall back to converting coordinates individually. This will be slower but also maybe not slow enough to really matter. I guess the question is whether if we do want to do this approach we should have a warning that this is happening.

This needs a test if we want to keep it.

[Cadair]

I'm not sure I love this change in behaviour. If we are going to have this shift, then we are going to have to raise a warning. Would it be better to do a longhand mean?

[codecov]

Codecov Report

❌ Patch coverage is 63.63636% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.83%. Comparing base (ff563b9) to head (74f2b87).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
reproject/mosaicking/wcs_helpers.py	63.63%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #584      +/-   ##
==========================================
- Coverage   91.06%   90.83%   -0.24%     
==========================================
  Files          30       30              
  Lines        1858     1865       +7     
==========================================
+ Hits         1692     1694       +2     
- Misses        166      171       +5     

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