Add FIR product to seviri_l2_grib reader

[strandgren]

Add support of reading additional products to the seviri_l2_grid reader.

• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

Merging #1405 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1405   +/-   ##
=======================================
  Coverage   90.56%   90.57%           
=======================================
  Files         228      228           
  Lines       33406    33431   +25     
=======================================
+ Hits        30254    30279   +25     
  Misses       3152     3152           

Impacted Files	Coverage Δ
satpy/readers/seviri_l2_grib.py	86.45% <ø> (ø)
satpy/readers/seviri_l2_bufr.py	93.90% <100.00%> (ø)
satpy/tests/reader_tests/test_mersi2_l1b.py	99.63% <0.00%> (+<0.01%)	⬆️
satpy/tests/test_composites.py	99.89% <0.00%> (+<0.01%)	⬆️
satpy/composites/__init__.py	82.80% <0.00%> (+0.01%)	⬆️

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[coveralls]

Coverage increased (+0.007%) to 90.572% when pulling 65ca36b on strandgren:feature_add_products_sev_grb into 12917ab on pytroll:master.

[pnuu]

Looks good to me. Thanks for the PR!

[sjoro]

Very good, just one minor comment/suggestion. Please add a link to the EUM Product Navigator (https://navigator.eumetsat.int/) to the references in seviri_l2_grib.py and seviri_l2_bufr.py.
Users can use the product navigator to find more info on EUM products, for example "what is the active fire classification? what do the numbers mean?".

[sjoro]

looks good to me!

[mraspaud]

LGTM

[mraspaud]

Thanks for the PR, and welcome to the Pytroll community!