Ensure lazyness of the olci nc reader

[mraspaud]

This PR fixes the olci nc reader so that the netcdf4 files for geolocaltion are not opened until necessary.

I really see this as a refactoring, so I didn't add any test, but I could if the reviewer thinks it's needed.

• Closes #xxxx
• Tests added
• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.94%. Comparing base (83678ac) to head (deeafbe).
Report is 397 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2819      +/-   ##
==========================================
- Coverage   95.95%   95.94%   -0.02%     
==========================================
  Files         368      366       -2     
  Lines       53692    53508     -184     
==========================================
- Hits        51518    51336     -182     
+ Misses       2174     2172       -2     

Flag	Coverage Δ
behaviourtests	4.04% <0.00%> (+0.01%)	⬆️
unittests	96.03% <100.00%> (-0.01%)	⬇️

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[coveralls]

Pull Request Test Coverage Report for Build 9499345069

Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.04%

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Totals
Change from base Build 9497248893:	0.0%
Covered Lines:	51565
Relevant Lines:	53691

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💛 - Coveralls

[pnuu]

If there is a good place for it and actual data are accessed in the tests, it'd be nice to have a check with satpy.tests.utils.CustomScheduler that there are no uninted computes. Otherwise LGTM.

[mraspaud]

No actual data is read i think, but good point for the future!