Fix data promotion in generic_image reader and satpy.composites.add_bands

[pnuu]

While figuring out why geo_color was float64, I found that both generic_image reader (used to read background images) and satpy.composites.add_bands() were mishandling the data. This fixes these two bugs in one go, as they both are triggered by BackgroundCompositor for certain cases.

Requires also pytroll/trollimage#186 for the geo_color to be fully fixed, the invert enhancement promotes data to float64 when the inversion parameters are given for each channel.

The generic_image reader tests were converted to pytest.

• Closes #xxxx
• Tests added
• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.07%. Comparing base (7eb17ab) to head (b8991c0).
Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2923   +/-   ##
=======================================
  Coverage   96.07%   96.07%           
=======================================
  Files         373      373           
  Lines       54479    54491   +12     
=======================================
+ Hits        52338    52352   +14     
+ Misses       2141     2139    -2     

Flag	Coverage Δ
behaviourtests	3.99% <0.00%> (-0.01%)	⬇️
unittests	96.17% <100.00%> (+<0.01%)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 11323677685

Details

• 197 of 197 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.004%) to 96.17%

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Totals
Change from base Build 11304823079:	0.004%
Covered Lines:	52586
Relevant Lines:	54680

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💛 - Coveralls

[mraspaud]

LGTM, thanks a lot for refactoring the tests!