support FIRG file-patterns coming from EUMETCast-Europe

[fwfichtner]

• Closes seviri_l2_grib-reader support for filename patterns of files coming from EUMETCast-Europe #2881
• Tests added
• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.97%. Comparing base (6860030) to head (abe48ae).
Report is 382 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2882      +/-   ##
==========================================
- Coverage   95.99%   95.97%   -0.02%     
==========================================
  Files         368      368              
  Lines       53985    53973      -12     
==========================================
- Hits        51821    51801      -20     
- Misses       2164     2172       +8     

Flag	Coverage Δ
behaviourtests	4.02% <ø> (+<0.01%)	⬆️
unittests	96.07% <ø> (-0.02%)	⬇️

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[coveralls]

Pull Request Test Coverage Report for Build 10365306296

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 8 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.01%) to 96.073%

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Files with Coverage Reduction	New Missed Lines	%
satpy/tests/utils.py	2	93.16%
satpy/tests/reader_tests/gms/test_gms5_vissr_l1b.py	3	98.67%
satpy/tests/reader_tests/gms/test_gms5_vissr_navigation.py	3	97.18%

Totals
Change from base Build 10303695673:	-0.01%
Covered Lines:	52030
Relevant Lines:	54157

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💛 - Coveralls

[ameraner]

LGTM, thanks for adding the path, good to see that it was the only thing needed to support the files!

[mraspaud]

LGTM