Update SLSTR calibration coefficients

[simonrp84]

The SLSTR calibration coefficients supplied in the reader are old and incorrect, this PR updates the coefficients to the latest ones supplied by EUM/ESA/Copernicus.

• Tests passed

[djhoese]

Just because I know people...should we make it possible to switch between them? I think you did this for other readers, right?

[simonrp84]

No, it should not be possible to switch between the old and new ones: The old ones were incorrect.

Also, the option already exists in the code (via user_calib kwarg) for the user to supply their own calibration values.

[djhoese]

Ah your description says "outdated" so I assumed they were corrected/updated, but not necessarily invalid before. Were these like pre-launch coefficients or something?

[simonrp84]

Heh, true. Have updated the description. Long story regarding why these values are needed: but the old values were from a previous iteration of the calibration efforts. New values are much more robust as they're derived from a) A longer time series and b) More validation sites.

[djhoese]

LGTM

[codecov]

Codecov Report

Merging #2056 (29d28ed) into main (01902ba) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2056   +/-   ##
=======================================
  Coverage   93.79%   93.79%           
=======================================
  Files         282      282           
  Lines       42171    42171           
=======================================
  Hits        39555    39555           
  Misses       2616     2616           

Flag	Coverage Δ
behaviourtests	4.74% <0.00%> (ø)
unittests	94.33% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/readers/slstr_l1b.py	97.29% <100.00%> (ø)

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[coveralls]

Coverage remained the same at 94.275% when pulling 29d28ed on simonrp84:update_slstr into 01902ba on pytroll:main.

[mraspaud]

LGTM