add rejection of frames that cannot be sky subtracted to avoid contamination of the good ones.

[KathleenLabrie]

Discussed at length with Chris. Seems to work on my test sequence that highlight the problem in the first place.

[codecov]

Codecov Report

Merging #219 (119f0a2) into release/3.0.x (2d899b3) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@                Coverage Diff                @@
##           release/3.0.x     #219      +/-   ##
=================================================
- Coverage          45.59%   45.56%   -0.04%     
=================================================
  Files                261      261              
  Lines              22305    22318      +13     
=================================================
- Hits               10171    10170       -1     
- Misses             12134    12148      +14     

Impacted Files	Coverage Δ
geminidr/core/primitives_preprocess.py	8.57% <0.00%> (-0.20%)	⬇️
geminidr/core/primitives_resample.py	44.15% <ø> (-0.37%)	⬇️

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[chris-simpson]

The code looks OK but I don't like the term "sky table" in the log, as this has no meaning for the user. (And it's not really a table, it's just a list but we store it as a Table object because that's the only way to propagate it.) I'd prefer something like

Sky frames could not be associated to any input frames. Sky subtraction will not be possible.

and

{ad.filename} does not have any associated sky frames so cannot be sky-subtracted, moving to "no_skies" stream

[use an f-string like the cool kidz]