Add flash age compositor for li instruments

[ClementLaplace]

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

• Add the flash_age compositor feature
• Document the code related to the feature
• Test the flash_age compositor

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@           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	Coverage Δ
behaviourtests	3.98% <0.00%> (-0.02%)	⬇️
unittests	96.16% <100.00%> (+<0.01%)	⬆️

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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%

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

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💛 - Coveralls

[ameraner]

Thanks for this first draft! I left a comment regarding the reference time.