Add true color RGB for FCI

[gerritholl]

Add a true color RGB for FCI. Refactor the AHI green corrector compositor into its own module, because FCI needs to use the same. The green band misses the chlorophyl peak, so instead the true color RGB uses a combination between VIS 0.5 and NIR 0.8.

• [ ] Closes #xxxx
• [ ] Tests added
• [ ] Fully documented

true_color_raw (was already supported):

The true_color with this PR looks like:

Without the green correction, the Congo Basin looks burnt down:

For the correction, I used a ratio of 0.93 VIS 0.5 and 0.07 NIR 0.8, such as suggested by @pnuu and reported here on Slack.

These images were generated from the FCI test data, version May 2022, simulating 2017-09-20. Repeat cycle 75, covering 12:20-12:29.

[codecov[bot]]

Codecov Report

Merging #2181 (718a27a) into main (7a30eb8) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2181   +/-   ##
=======================================
  Coverage   94.13%   94.14%           
=======================================
  Files         293      294    +1     
  Lines       45079    45103   +24     
=======================================
+ Hits        42437    42461   +24     
  Misses       2642     2642           

Flag	Coverage Δ
behaviourtests	4.68% <0.00%> (-0.01%)	:arrow_down:
unittests	94.79% <100.00%> (+<0.01%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/composites/ahi.py	100.00% <100.00%> (ø)
satpy/composites/spectral.py	100.00% <100.00%> (ø)
satpy/tests/reader_tests/test_abi_l2_nc.py	100.00% <0.00%> (ø)
satpy/readers/abi_base.py	94.52% <0.00%> (+0.07%)	:arrow_up:

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[coveralls]

Coverage increased (+0.003%) to 94.742% when pulling 718a27a1958e963e4d0f5f7b24eb70ba4bed784c on gerritholl:fci-true-color into 7a30eb871237b6e3bb997998c65f2c9c1d4e395e on pytroll:main.

[gerritholl]

the vis_08 channel is too bright, so that a factor of 0.8 should be applied to get correct radiances/reflectances

Although I don't think tuning based on buggy test data is the most productive, I've tried to replace the 0.07 by 0.056 all else being equal, I get:

which is very slightly darker and not an improvement IMHO, so I think further tuning should be done when we have real data.

[simonrp84]

LGTM!

[gerritholl]

This fails only for the experimental run now, which, as I understand it, is allowed. So the PR is ready as far as I'm concerned.