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[bot]]

Codecov Report

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

Project coverage is 95.94%. Comparing base (3c7938c) to head (deeafbe). Report is 23 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2819 +/- ## ======================================= Coverage 95.94% 95.94% ======================================= Files 366 366 Lines 53504 53508 +4 ======================================= + Hits 51332 51336 +4 Misses 2172 2172 ``` | [Flag](https://app.codecov.io/gh/pytroll/satpy/pull/2819/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/2819/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `4.04% <0.00%> (-0.01%)` | :arrow_down: | | [unittests](https://app.codecov.io/gh/pytroll/satpy/pull/2819/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `96.03% <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 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

[mraspaud]

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