Change crop_gribs to operate on files as they arrive

[douglatornell]

Use watchdog to detect when grib files are moved into place and crop them immediately instead of waiting until collect_weather finishes. crop_gribs is now launched concurrently with collect_weather.

[douglatornell]

Tested successfully on skookum for 3aug23 18Z forecast:

• grib files were cropped concurrently with downloads
• after_collect_weather() launched collect_weather 00 2.5km and crop_gribs 00 2023-08-04 as expected

[codecov[bot]]

Codecov Report

Merging #191 (078a811) into main (b684dc3) will increase coverage by 0.17%. Report is 2 commits behind head on main. The diff coverage is 95.55%.

@@            Coverage Diff             @@
##             main     #191      +/-   ##
==========================================
+ Coverage   77.41%   77.58%   +0.17%     
==========================================
  Files         133      133              
  Lines       18483    18601     +118     
  Branches     1953     1967      +14     
==========================================
+ Hits        14309    14432     +123     
+ Misses       4119     4111       -8     
- Partials       55       58       +3     

Flag	Coverage Δ
unittests	77.58% <95.55%> (+0.17%)	:arrow_up:

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Files Changed	Coverage Δ
nowcast/workers/crop_gribs.py	93.06% <87.50%> (+12.18%)	:arrow_up:
tests/workers/test_crop_gribs.py	99.53% <99.09%> (-0.47%)	:arrow_down:
nowcast/next_workers.py	98.00% <100.00%> (-0.01%)	:arrow_down:
tests/test_next_workers.py	99.74% <100.00%> (-0.01%)	:arrow_down:

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