Add enhanced (more natural) version of natural colors composite

[pnuu]

This PR adds an enhanced version of natural colors composite that produces more natural colors for the clouds.

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

[pnuu]

The composite and compositor names are just placeholders until someone comes up with a better name.

Also some help is needed for getting proper channel weights for other instruments than SEVIRI. These weights now default to those shown in https://drmarkh.wordpress.com/2014/07/11/plotting-mcidas-and-sort-of-natural-colour-images/ for SEVIRI.

[mraspaud]

Is this ready for merge @pnuu (appart from the conflicts of course) ?

[pnuu]

No, not ready. The channel weights needs tuning, and that's something I don't have patience to do 😁

[mraspaud]

we could merge this and have the weight tuning in another PR ?

[pnuu]

Sure. I'll fix the conflicts.

[mraspaud]

thanks

[ghost]

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[coveralls]

Coverage increased (+0.01%) to 89.473% when pulling 9514b24 on pnuu:feature-enhanced-natural-colors into 6c11402 on pytroll:master.

[coveralls]

Coverage increased (+0.02%) to 89.476% when pulling 9514b24 on pnuu:feature-enhanced-natural-colors into 6c11402 on pytroll:master.

[codecov]

Codecov Report

Merging #606 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #606      +/-   ##
==========================================
+ Coverage   89.46%   89.47%   +0.01%     
==========================================
  Files         198      198              
  Lines       29167    29210      +43     
==========================================
+ Hits        26093    26136      +43     
  Misses       3074     3074              

Impacted Files	Coverage Δ
satpy/tests/compositor_tests/__init__.py	100.00% <ø> (ø)
satpy/composites/__init__.py	78.76% <100.00%> (+0.39%)	⬆️
satpy/tests/test_composites.py	100.00% <100.00%> (ø)

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[pnuu]

The natural_enh composite added in this PR:

The already existing natural_color composite:

[djhoese]

I see this is merged already but thoughts on a generic weighted compositor? Or maybe an add-on to the generic one which takes a tuple of weights for each band? Also this name may cause a lot of confusion since I think the NOAA imagery teams uses enhanced natural color as the name for one of their composites (a true color).