Make it possible to set fill value when reading images

[pnuu]

This PR makes it possible to set fill value when reading for LA and RGBA images with generic_image reader. When this is done, the A band is dropped. Previously RGBA images were converted to float, which caused problems in https://github.com/pytroll/satpy/issues/2897 .

• [x] Closes #2928
• [x] Tests added
• [ ] Fully documented

[pnuu]

This is still a very rough draft, so comments are welcome.

[codecov[bot]]

Codecov Report

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

Project coverage is 96.09%. Comparing base (d62b5d2) to head (6aec8e2). Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2940      +/-   ##
==========================================
- Coverage   96.11%   96.09%   -0.03%     
==========================================
  Files         377      377              
  Lines       55067    55090      +23     
==========================================
+ Hits        52929    52940      +11     
- Misses       2138     2150      +12     

Flag	Coverage Δ
behaviourtests	3.94% <0.00%> (-0.01%)	:arrow_down:
unittests	96.19% <100.00%> (-0.03%)	:arrow_down:

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

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[coveralls]

Pull Request Test Coverage Report for Build 11402206297

Details

• 30 of 30 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 96.203%

---

Totals
Change from base Build 11398631499:	0.001%
Covered Lines:	53184
Relevant Lines:	55283

---

💛 - Coveralls

[strandgren]

Thanks for working on this, I can confirm that by setting reader_kwargs={"set_fill_value": 255} the issue I originally had in https://github.com/pytroll/satpy/issues/2897 is resolved :)

Would it be possible though to read a PNG image with alpha channel and store it again with Scene.save_datasets and retain the alpha channel? So far I have not found a way.

[djhoese]

set_fill_value -> fill_value?

Seems odd to me to have a verb in a variable name. Then again I am probably guilty of doing use_x_feature=True or filter_x=True. But in this case the action you're doing is the variable you're specifying.