Add the possibility to include scale and offset in geotiffs

[mraspaud]

This PR makes it possible to pass dynamically computed scale and offset to the geotiff writer

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

[coveralls]

Coverage increased (+0.3%) to 86.099% when pulling 6ab34bd on mraspaud:feature-scale-offset-tags into 1a00661 on pytroll:master.

[codecov]

Codecov Report

Merging #948 into master will increase coverage by 0.26%.
The diff coverage is 91.66%.

@@            Coverage Diff            @@
##           master    #948      +/-   ##
=========================================
+ Coverage   85.83%   86.1%   +0.26%     
=========================================
  Files         172     174       +2     
  Lines       26222   26531     +309     
=========================================
+ Hits        22508   22844     +336     
+ Misses       3714    3687      -27

Impacted Files	Coverage Δ
satpy/writers/utils.py	100% <100%> (ø)	⬆️
satpy/writers/geotiff.py	90.56% <100%> (+0.18%)	⬆️
satpy/tests/writer_tests/test_geotiff.py	94.84% <100%> (+0.59%)	⬆️
satpy/tests/writer_tests/test_ninjotiff.py	97.72% <100%> (+0.1%)	⬆️
satpy/writers/ninjotiff.py	59.37% <71.42%> (+12.4%)	⬆️
satpy/tests/compositor_tests/test_viirs.py	98.71% <0%> (-0.59%)	⬇️
satpy/composites/ahi.py	100% <0%> (ø)	⬆️
satpy/tests/reader_tests/test_ami_l1b.py	94.73% <0%> (ø)
satpy/readers/ami_l1b.py	97.32% <0%> (ø)
satpy/tests/compositor_tests/__init__.py	99.57% <0%> (+0.02%)	⬆️
... and 6 more

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