Add 'seadas_l2' reader for 'chlor_a' product

[djhoese]

Attempt at reading some files that came across my desk that are created by SEADAS and are created from either MODIS or VIIRS data.

• Tests added
• Fully documented

[djhoese]

Last thing I need to do is finalize the enhancement.

[codecov]

Codecov Report

Merging #1894 (5a240be) into main (ef301b0) will increase coverage by 0.09%.
The diff coverage is 98.18%.

@@            Coverage Diff             @@
##             main    #1894      +/-   ##
==========================================
+ Coverage   93.29%   93.38%   +0.09%     
==========================================
  Files         273      275       +2     
  Lines       40460    40732     +272     
==========================================
+ Hits        37748    38039     +291     
+ Misses       2712     2693      -19     

Flag	Coverage Δ
behaviourtests	4.81% <0.00%> (+0.09%)	⬆️
unittests	93.95% <98.18%> (+0.09%)	⬆️

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

Impacted Files	Coverage Δ
satpy/readers/seadas_l2.py	97.82% <97.82%> (ø)
satpy/tests/reader_tests/test_seadas_l2.py	98.43% <98.43%> (ø)
satpy/tests/reader_tests/test_vii_l2_nc.py	93.93% <0.00%> (-6.07%)	⬇️
satpy/tests/reader_tests/test_vii_l1b_nc.py	97.18% <0.00%> (-2.82%)	⬇️
satpy/tests/reader_tests/test_nwcsaf_msg.py	96.63% <0.00%> (-1.69%)	⬇️
satpy/readers/li_l2.py	11.66% <0.00%> (-1.67%)	⬇️
satpy/tests/test_config.py	93.84% <0.00%> (-0.77%)	⬇️
satpy/resample.py	80.03% <0.00%> (-0.25%)	⬇️
satpy/readers/seviri_l1b_nc.py	67.29% <0.00%> (-0.21%)	⬇️
satpy/readers/ghrsst_l3c_sst.py	7.04% <0.00%> (-0.21%)	⬇️
... and 206 more

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[coveralls]

Coverage increased (+0.1%) to 93.905% when pulling 5a240be on djhoese:feature-seadas-l2 into ef301b0 on pytroll:main.

[mraspaud]

LGTM, just one small suggestion