Ignore alpha when adding luminance in Sandwich compositor

[mherbertson]

This updates the Sandwich compositor to ignore the alpha band when modifying.

• Tests added

[djhoese]

Wow, what a simple solution. Nice job and thanks for making the PR!

Are there tests for this compositor already? Could they be modified or added onto to check for this fix?

[mherbertson]

That's a good idea. I think the existing check actually just did the first three bands 'RGB'. So adding one to check the alpha stays intact makes sense. I'll see what I can come up with.

[mherbertson]

Existing test updated to confirm alpha band remains intact.

[codecov]

Codecov Report

Merging #2080 (0a97b74) into main (683df59) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2080   +/-   ##
=======================================
  Coverage   93.79%   93.79%           
=======================================
  Files         283      283           
  Lines       42281    42284    +3     
=======================================
+ Hits        39659    39662    +3     
  Misses       2622     2622           

Flag	Coverage Δ
behaviourtests	4.73% <0.00%> (-0.01%)	⬇️
unittests	94.36% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
satpy/composites/__init__.py	89.92% <100.00%> (ø)
satpy/tests/test_composites.py	100.00% <100.00%> (ø)

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[coveralls]

Coverage increased (+0.0004%) to 94.3% when pulling 0a97b74 on mherbertson:update-sandwich-compositor into 683df59 on pytroll:main.

[mherbertson]

Not sure what to do about the CodeFactor failure.

[mraspaud]

@mherbertson that usually means that some of the functions/classes in that modules are too convoluted, and should thus be refactored to smaller functions/classes. But in the changes you made, I don't really see how to make things simpler, do you?

[mherbertson]

@mraspaud Thanks for the explanation! I don't think there's much I can do to simplify.

[mraspaud]

LGTM