Convert Sentinel-2 MSI sensor name to lowercase in the reader YAML config file and add support for "counts" calibration

[yukaribbba]

This PR has two purposes:

1. The current sensor name in YAML is "MSI", which triggers a minor warning Inconsistent sensor/satellite input in pyspectral since the latter one stores sensor name in lowercase. So it's replace by lowercase.
2. Add "counts" calibration just like other sensors.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.93%. Comparing base (24109a1) to head (4faccbb).
Report is 20 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2779   +/-   ##
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  Coverage   95.92%   95.93%           
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  Files         377      377           
  Lines       53535    53541    +6     
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+ Hits        51356    51362    +6     
  Misses       2179     2179           

Flag	Coverage Δ
behaviourtests	4.10% <0.00%> (-0.01%)	⬇️
unittests	96.02% <100.00%> (+<0.01%)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 8721657075

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 0.0%

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Totals
Change from base Build 8626996485:	0.0%
Covered Lines:	0
Relevant Lines:	0

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💛 - Coveralls

[djhoese]

I'm surprised the sensor is defined per dataset, but that's not your doing so don't worry about it. This looks good to me, but I'd like someone else who has more experience with this data hit the merge button.

@mraspaud ?

[mraspaud]

LGTM, thanks for making the counts available and making tests for it.