Prepare for channel-specific SEVIRI calibration

[sfinkens]

Prepare for channel-specific calibration (#2599) by using satpy.readers.utils.CalibrationCoefficientPicker internally to select coefficients. No changes to public interfaces. Next steps in a future PR:

• Add calib_wishlist reader kwarg
• Deprecate calib_mode and ext_calib_coefs kwargs
• Add calibration coefficients and mode to dataset metadata (Make SEVIRI calibration more transparent to the user #2569). This is easier now, because CalibrationCoefficientPicker provides mode alongside coefficients.

• Tests added

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.09%. Comparing base (fa56be5) to head (5ab316e).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2997   +/-   ##
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  Coverage   96.08%   96.09%           
=======================================
  Files         377      377           
  Lines       55155    55231   +76     
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+ Hits        52997    53073   +76     
  Misses       2158     2158           

Flag	Coverage Δ
behaviourtests	3.93% <0.00%> (-0.01%)	⬇️
unittests	96.18% <100.00%> (+<0.01%)	⬆️

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[sfinkens]

start behave test

[coveralls]

Pull Request Test Coverage Report for Build 11970487783

Details

• 150 of 150 (100.0%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 96.198%

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Totals
Change from base Build 11936881763:	0.005%
Covered Lines:	53317
Relevant Lines:	55424

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💛 - Coveralls

[sfinkens]

Regarding the CodeScene issue about seviri_base.py being too complex: I could move the calibration parts into a separate file.