Update seadas_l2 reader to handle alternative NetCDF file format

[djhoese]

The SEADAS software being used by @kathys and Liam Gumley can produce HDF4 files (already supported) or NetCDF files. This PR adds support for the new files which have similar but slightly different structures.

Related Polar2Grid issue: ssec/polar2grid#457

• Closes Change support for SeaDAS product from HDF4 to NetCDF4 Output Format ssec/polar2grid#457
• Tests added
• Fully documented

[codecov]

Codecov Report

Merging #2209 (fc652d7) into main (4baae95) will increase coverage by 0.00%.
The diff coverage is 98.05%.

@@           Coverage Diff           @@
##             main    #2209   +/-   ##
=======================================
  Coverage   94.14%   94.14%           
=======================================
  Files         294      294           
  Lines       45103    45178   +75     
=======================================
+ Hits        42461    42534   +73     
- Misses       2642     2644    +2     

Flag	Coverage Δ
behaviourtests	4.67% <0.00%> (-0.01%)	⬇️
unittests	94.79% <98.05%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/readers/seadas_l2.py	96.84% <96.66%> (-1.28%)	⬇️
satpy/tests/reader_tests/test_seadas_l2.py	99.07% <100.00%> (+0.40%)	⬆️

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[coveralls]

Coverage increased (+0.004%) to 94.747% when pulling fc652d7 on djhoese:feature-seadas-nc into 4baae95 on pytroll:main.

[mraspaud]

Nice job, just a question regarding style

[mraspaud]

LGTM