Return uncertainty and mask from combine function

[mwcraig]

This fixes #774 by ensuring that a mask and uncertainty is returned from the combine function.

[codecov]

Codecov Report

Merging #775 (f7cf5c6) into main (91f2c46) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #775      +/-   ##
==========================================
+ Coverage   95.94%   96.01%   +0.07%     
==========================================
  Files          30       30              
  Lines        3922     3942      +20     
==========================================
+ Hits         3763     3785      +22     
+ Misses        159      157       -2     

Impacted Files	Coverage Δ
ccdproc/tests/test_memory_use.py	89.28% <ø> (-0.72%)	⬇️
ccdproc/combiner.py	94.02% <100.00%> (+0.90%)	⬆️
ccdproc/tests/test_combiner.py	100.00% <100.00%> (ø)

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