Handle missing Neah Bay sea surface height obs by symlink to forecast file

[douglatornell]

This moves handling of the exception case of persisting forecast values as observations when obs are missing/incomplete from upload_forcing to make_ssh_files. Doing so should avoid the symlink/file-exists race conditions that often arises when upload_forcing tries to create the symlink.

[codecov]

Codecov Report

Merging #114 (071d3c7) into main (7066ba8) will decrease coverage by 0.36%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##             main     #114      +/-   ##
==========================================
- Coverage   73.82%   73.45%   -0.37%     
==========================================
  Files         125      125              
  Lines       16278    16316      +38     
  Branches     1164     1168       +4     
==========================================
- Hits        12017    11985      -32     
- Misses       4206     4284      +78     
+ Partials       55       47       -8     

Flag	Coverage Δ
unittests	73.45% <89.47%> (-0.37%)	⬇️

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Impacted Files	Coverage Δ
tests/workers/test_make_ssh_file.py	83.21% <88.46%> (-16.79%)	⬇️
nowcast/workers/make_ssh_files.py	26.29% <91.66%> (-18.49%)	⬇️

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[douglatornell]

Worked as expected on 15oct22 for a day with complete obs (14oct) - this is normal operations confirmation.

obs -> fcts symlink was created as expected on 16oct22 when 15oct22 obs collected for forecast2 run were missing some values. Values were also missing later when the file for the forecast run was collected, so the symlink was not replaced with an obs file. This is full obs failure operations confirmation.

Not yet confirmed: operation when obs are missing at forecast2 time but available at forecast time. Vice versa is hypothetically possible too, I guess, but latter is a variant on the missing obs scenario that was confirmed on 16oct22.