Add flash age compositor for li instruments

[ClementLaplace]

This Merge request is there to create the flash age compositor related to the li instrument.

• [x] Add the flash_age compositor feature
• [x] Document the code related to the feature
• [x] Test the flash_age compositor

[codecov[bot]]

Codecov Report

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

Project coverage is 96.07%. Comparing base (084b114) to head (3edf893). Report is 131 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2895 +/- ## ======================================== Coverage 96.06% 96.07% ======================================== Files 370 375 +5 Lines 54320 54474 +154 ======================================== + Hits 52185 52335 +150 - Misses 2135 2139 +4 ``` | [Flag](https://app.codecov.io/gh/pytroll/satpy/pull/2895/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/2895/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `3.98% <0.00%> (-0.02%)` | :arrow_down: | | [unittests](https://app.codecov.io/gh/pytroll/satpy/pull/2895/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `96.16% <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 10700852657

Details

• 18 of 34 (52.94%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on add_flash_age_compositor at 96.124%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
satpy/composites/lightning.py	18	34	52.94%
<!--	Total:	18	34	52.94%	-->

Totals
Change from base Build 10654375136:	96.1%
Covered Lines:	52426
Relevant Lines:	54540

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💛 - Coveralls