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.

• [x] Tests added

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2767 +/- ## ======================================= Coverage 95.92% 95.92% ======================================= Files 377 377 Lines 53489 53535 +46 ======================================= + Hits 51310 51356 +46 Misses 2179 2179 ``` | [Flag](https://app.codecov.io/gh/pytroll/satpy/pull/2767/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | Coverage Δ | | |---|---|---| | [behaviourtests](https://app.codecov.io/gh/pytroll/satpy/pull/2767/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `4.10% <0.00%> (-0.01%)` | :arrow_down: | | [unittests](https://app.codecov.io/gh/pytroll/satpy/pull/2767/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `96.02% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll#carryforward-flags-in-the-pull-request-comment) to find out more.

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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