Add single variable-hour file cropping to crop_gribs worker

[douglatornell]

Allow the crop_gribs worker to crop a specific forecast variable for a specific forecast hour. This is primarily intended for recovery from crop_gribs stalls when only 1 or a few files need to be cropped to finish processing. The feature is added through the use of optional --var-hour and --var arguments. Test cases for this new functionality are included.

[codecov[bot]]

Codecov Report

Merging #195 (b0e9b38) into main (6f60191) will increase coverage by 0.06%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #195      +/-   ##
==========================================
+ Coverage   77.58%   77.65%   +0.06%     
==========================================
  Files         133      133              
  Lines       18601    18655      +54     
  Branches     1967     1969       +2     
==========================================
+ Hits        14432    14486      +54     
  Misses       4111     4111              
  Partials       58       58              

Flag	Coverage Δ
unittests	77.65% <100.00%> (+0.06%)	:arrow_up:

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Files Changed	Coverage Δ
nowcast/workers/crop_gribs.py	93.91% <100.00%> (+0.84%)	:arrow_up:
tests/workers/test_crop_gribs.py	99.60% <100.00%> (+0.07%)	:arrow_up:

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[douglatornell]

Successfully tested in production on skookum for uncropped PRATE_Sfc hour 001 file in 18aug23 18Z forecast.