Add the ocean color product to olci

[mraspaud]

This is basically the same as true_color with a higher gamma

• Tests added
• Tests passed
• Passes git diff origin/develop **/*py | flake8 --diff

[coveralls]

Coverage remained the same at 65.487% when pulling 86123ac on feature-olci-ocean-color into 8a4d74f on develop.

[codecov]

Codecov Report

Merging #294 into master will decrease coverage by 0.35%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #294      +/-   ##
==========================================
- Coverage   65.86%   65.51%   -0.36%     
==========================================
  Files         118      117       -1     
  Lines       14321    14273      -48     
==========================================
- Hits         9433     9351      -82     
- Misses       4888     4922      +34

Impacted Files	Coverage Δ
satpy/readers/hrit_goes.py	27.32% <0%> (-23.36%)	⬇️
satpy/readers/hrit_msg.py	24.88% <0%> (-0.35%)	⬇️
satpy/tests/reader_tests/__init__.py	96.29% <0%> (-0.14%)	⬇️
satpy/tests/reader_tests/test_hrit_goes.py
satpy/readers/yaml_reader.py	85.07% <0%> (+0.03%)	⬆️
satpy/readers/file_handlers.py	88.67% <0%> (+0.44%)	⬆️
satpy/readers/hrit_base.py	81.73% <0%> (+5.73%)	⬆️

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