Add a compositor to mask visible night

[mraspaud]

My use case for the true color composite is often to put it on top of another composite that fills in the night part of the image.
For that a straightforward solution is to use the daynightcompositor. However, I'm never really satisfied because it in a compromise between having a large termiator area or loosing bright spots from true color on the night side.

This PR proposes a simple thresholding from the image channel to complement the opacity gradient.

Feedback on this idea is welcome.

• Closes #xxxx
• Tests added
• Fully documented

Examples:

• FCI

With another composite behind

• VIIRS
  

• an example configuration for this composite:

  true_color_day:
    compositor: !!python/name:satpy.composites.MaskVisibleNightCompositor
    standard_name: image_ready
    lim_low: 88.0
    prerequisites:
      - name: true_color

[codecov]

Codecov Report

Attention: Patch coverage is 14.54545% with 47 lines in your changes missing coverage. Please review.

Project coverage is 96.08%. Comparing base (3a208c0) to head (2c87c5a).
Report is 26 commits behind head on main.

Files with missing lines	Patch %	Lines
satpy/composites/__init__.py	14.54%	47 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3102      +/-   ##
==========================================
- Coverage   96.16%   96.08%   -0.08%     
==========================================
  Files         389      389              
  Lines       56418    56473      +55     
==========================================
+ Hits        54256    54264       +8     
- Misses       2162     2209      +47     

Flag	Coverage Δ
behaviourtests	3.85% <14.54%> (+0.01%)	⬆️
unittests	96.18% <14.54%> (-0.08%)	⬇️

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[mraspaud]

An example for the cloud phase composite

[coveralls]

Pull Request Test Coverage Report for Build 14337460508

Details

• 8 of 55 (14.55%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.08%) to 96.192%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
satpy/composites/init.py	8	55	14.55%

Totals
Change from base Build 14322426188:	-0.08%
Covered Lines:	54516
Relevant Lines:	56674

---

💛 - Coveralls

[TAlonglong]

Oh, this looks really nice.