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 #2897 .

• Closes Allow generic_image reader to set fill value on reading #2928
• Tests added
• Fully documented

[pnuu]

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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%)	⬇️
unittests	96.19% <100.00%> (-0.03%)	⬇️

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[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%

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

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💛 - Coveralls