Feature mitiff palette

[TAlonglong]

This PR adds the possibility to write mitiff palette images

• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy
• Fully documented

[codecov]

Codecov Report

Merging #947 into master will increase coverage by 0.03%.
The diff coverage is 97.08%.

@@            Coverage Diff             @@
##           master     #947      +/-   ##
==========================================
+ Coverage   86.61%   86.64%   +0.03%     
==========================================
  Files         175      175              
  Lines       26837    26925      +88     
==========================================
+ Hits        23244    23330      +86     
- Misses       3593     3595       +2

Impacted Files	Coverage Δ
satpy/tests/writer_tests/test_mitiff.py	98.24% <100%> (+0.33%)	⬆️
satpy/writers/mitiff.py	91.64% <94.73%> (+0.41%)	⬆️

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[coveralls]

Coverage increased (+0.03%) to 86.647% when pulling d26a1b3 on TAlonglong:feature-mitiff-palette into 42d1c6a on pytroll:master.

[coveralls]

Coverage increased (+0.02%) to 86.633% when pulling a2375bc on TAlonglong:feature-mitiff-palette into 42d1c6a on pytroll:master.

[mraspaud]

I don't know the format, but from what I understand it LGTM. Nice with the small refactoring also.