Use flags from file when available in OLCI NC reader

[mraspaud]

This PR adds support for in-file flags masks and meanings in the olci-nc reader.

• Tests added

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.92%. Comparing base (5ffe15e) to head (3aa2f79).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2767   +/-   ##
=======================================
  Coverage   95.92%   95.92%           
=======================================
  Files         377      377           
  Lines       53489    53535   +46     
=======================================
+ Hits        51310    51356   +46     
  Misses       2179     2179           

Flag	Coverage Δ
behaviourtests	4.10% <0.00%> (-0.01%)	⬇️
unittests	96.02% <100.00%> (+<0.01%)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 8432424384

Details

• 48 of 48 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 96.008%

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Totals
Change from base Build 8350021041:	0.003%
Covered Lines:	51228
Relevant Lines:	53358

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💛 - Coveralls