Fix nwcsaf_geo start time to be nominal time

[mraspaud]

NWCSAF GEO file contain a different time for start of time coverage and nominal time. This PR makes the reader choose the nominal time when available.

• Tests added

[codecov]

Codecov Report

Merging #2635 (e43f3fa) into main (1e0cf20) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2635   +/-   ##
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  Coverage   95.20%   95.20%           
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  Files         354      354           
  Lines       51368    51374    +6     
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+ Hits        48906    48912    +6     
  Misses       2462     2462           

Flag	Coverage Δ
behaviourtests	4.24% <0.00%> (-0.01%)	⬇️
unittests	95.83% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
satpy/readers/nwcsaf_nc.py	82.21% <100.00%> (+0.21%)	⬆️
satpy/tests/reader_tests/test_nwcsaf_nc.py	100.00% <100.00%> (ø)

[coveralls]

Pull Request Test Coverage Report for Build 6825204541

• 10 of 10 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 95.783%

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Totals
Change from base Build 6816647681:	0.001%
Covered Lines:	49038
Relevant Lines:	51197

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💛 - Coveralls

[djhoese]

Another way of looking at this PR is that it is fixing the reader to follow the standards for Satpy metadata defined here: https://satpy.readthedocs.io/en/latest/reading.html#time-metadata

I assume by your try/except that nominal time isn't always available? If that is the case then LGTM.

[mraspaud]

Another way of looking at this PR is that it is fixing the reader to follow the standards for Satpy metadata defined here: https://satpy.readthedocs.io/en/latest/reading.html#time-metadata

I assume by your try/except that nominal time isn't always available? If that is the case then LGTM.

Yes exactly. This reader can handle files from two different software, where only one of them provides a nominal time. The other one doesn't need it as its about polar data.