Add possibility to override area_def in gerb reader

[pdebuyl]

This is necessary because some GERB files do not have the sub-satellite longitude as a parameter.

The proposed fix is to add a reader_kwargs for the area definition of the gerb reader. The possibilities for overriding are msg_seviri_fes_9km or msg_seviri_iodc_9km. Altough unreleased, some data might use IODC at 41.5 degrees instead of 45.5 degrees (see https://usc.tools.eumetsat.int/reports//missions/geo_ssp_iodc.html for the history of sub-satellite points) but I'll have the required attribute added to GERB L2 data for upcoming releases.

• Tests added
• Fully documented

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.33%. Comparing base (d035d04) to head (aaf4882).
⚠️ Report is 247 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3220   +/-   ##
=======================================
  Coverage   96.32%   96.33%           
=======================================
  Files         463      463           
  Lines       58181    58189    +8     
=======================================
+ Hits        56041    56054   +13     
+ Misses       2140     2135    -5     

Flag	Coverage Δ
behaviourtests	3.64% <0.00%> (-0.02%)	⬇️
unittests	96.42% <100.00%> (+<0.01%)	⬆️

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[pdebuyl]

Hello, the Python 3.11 test on windows seems unrelated to this PR, should I restart the tests?

[djhoese]

but I'll have the required attribute added to GERB L2 data for upcoming releases

Does this mean this workaround will no longer be needed in the next release of the GERB L2 software?

[coveralls]

Pull Request Test Coverage Report for Build 17442455296

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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Details

• 10 of 10 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 96.43%

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Totals
Change from base Build 17329287279:	0.001%
Covered Lines:	56379
Relevant Lines:	58466

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💛 - Coveralls

[pdebuyl]

For the next official release of GERB data. Currently, we have the "Edition 1" release where only "GERB files" (so not the GERB like files that lack the attribute) are present, but also the near real time and some (one released dataset called V100) datasets containing GERB + GERB-like but those latter files are unreadable currently.

[pdebuyl]

But indeed at the next release I would like to have this issue resolved within the HDF5 files of GERB and GERB-like directly.

[djhoese]

I guess what I'm leading to is should the changes made here just stay in your local copy of Satpy for reading these files? Or should this PR be merged to make it more available to others and/or easier to install in your systems?

[pdebuyl]

I made the PR because one scientist at a partner institution had trouble with those "GERB-like" files. I would like to have it merged :-)

[pdebuyl]

Both suggestions implemented.

[djhoese]

Looks good. Thanks!

[pdebuyl]

🥳 thanks