Add additional Sentinel 3 OLCI 2 datasets

[johannesocean]

This PR adds the following datasets to the olci l2 reader:

iop_nn (cdm_absorption_coefficient)
trsp (diffuse_attenuation_coefficient)
tsm_nn (total_suspended_matter_concentration)

• Tests passed
• Add your name to AUTHORS.md if not there already

[coveralls]

Coverage increased (+0.02%) to 87.29% when pulling 42237e6 on JohannesSMHI:feature-new-olci_2-datasets into 2f9501e on pytroll:master.

[codecov]

Codecov Report

Merging #1035 into master will increase coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1035      +/-   ##
==========================================
+ Coverage   87.29%   87.42%   +0.13%     
==========================================
  Files         188      188              
  Lines       28742    29033     +291     
==========================================
+ Hits        25089    25383     +294     
+ Misses       3653     3650       -3

Impacted Files	Coverage Δ
satpy/tests/writer_tests/test_cf.py	98.64% <0%> (+0.14%)	⬆️
satpy/tests/reader_tests/test_abi_l1b.py	97.41% <0%> (+0.19%)	⬆️
satpy/tests/reader_tests/test_glm_l2.py	96.42% <0%> (+1.55%)	⬆️
satpy/tests/reader_tests/test_abi_l2_nc.py	96.33% <0%> (+2.48%)	⬆️
satpy/writers/cf_writer.py	93.19% <0%> (+2.51%)	⬆️
satpy/readers/abi_base.py	91.6% <0%> (+3.05%)	⬆️

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[mraspaud]

LGTM