read the "elevation" variable in slstr_l1b

[ghislainp]

The slstr_l1b drivers now can read the "elevation" variable along with latitude and longitude in the geodetic files.

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

[codecov]

Codecov Report

Merging #1989 (6eb2936) into main (15f727b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1989   +/-   ##
=======================================
  Coverage   93.54%   93.54%           
=======================================
  Files         279      279           
  Lines       41322    41322           
=======================================
  Hits        38654    38654           
  Misses       2668     2668           

Flag	Coverage Δ
behaviourtests	4.78% <ø> (ø)
unittests	94.07% <ø> (ø)

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[coveralls]

Coverage remained the same at 94.005% when pulling 6eb2936 on ghislainp:master into 15f727b on pytroll:main.

[mraspaud]

Thanks a lot for adding the elevation :) LGTM