Add more datasets to IASI L2 reader

[pnuu]

This PR adds datasets from /INFO/ and /Maps/ groups to the IASI L2 reader.

Also the IASI L2 tests were converted to pytest.

• Tests added

[codecov]

Codecov Report

Attention: Patch coverage is 97.87234% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.13%. Comparing base (91e758a) to head (1d459e7).
Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
satpy/readers/iasi_l2.py	96.87%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3059      +/-   ##
==========================================
+ Coverage   96.11%   96.13%   +0.01%     
==========================================
  Files         383      383              
  Lines       55673    55719      +46     
==========================================
+ Hits        53511    53564      +53     
+ Misses       2162     2155       -7     

Flag	Coverage Δ
behaviourtests	3.88% <0.00%> (-0.01%)	⬇️
unittests	96.22% <97.87%> (+0.01%)	⬆️

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