Add initial support for EUMETSAT's climatological calibration

[simonrp84]

EUMETSAT are working on long-term calibration coefficients for SEVIRI, which will be available as external files specifying the gain and offset for each satellite over their lifetime.

This PR adds initial support for these calibration coefficients as an additional calibration method, EUMCLIM. Currently, I've only added support for the native reader, as this is the data format for historical images on the data store and eoportal. I will also add support to the HRIT and netCDF readers at a later point, as this should be reasonably straightforward.

This is a draft for now as I need to add the docs.

• [x] Tests added
• [x] Fully documented

[codecov[bot]]

Codecov Report

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

Project coverage is 94.69%. Comparing base (9d72326) to head (53bfccb). Report is 2904 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2375   +/-   ##
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  Coverage   94.68%   94.69%           
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  Files         329      329           
  Lines       48616    48707   +91     
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+ Hits        46030    46121   +91     
  Misses       2586     2586           

Flag	Coverage Δ
behaviourtests	4.42% <0.96%> (-0.01%)	:arrow_down:
unittests	95.31% <100.00%> (+<0.01%)	:arrow_up:

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[coveralls]

Coverage: 95.275% (+0.009%) from 95.266% when pulling 53bfccb42690ccc7222cbd031b74225b309a763e on simonrp84:sev_eum_climate_cal into 9d7232642fc8495be1b464c231b9e1fc3951c7c2 on pytroll:main.

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