Fix MERSI-2 natural_color composite using the wrong band for sharpening

[djhoese]

The ratio sharpening compositor uses the optional (4th) prerequisite as the high resolution band. For MERSI-2's natural_color composite I had the high/low res green bands swapped. Band 4 is the high res and should be optional, band 15 is the low res and should be the required. This should have minimal different on the red and blue bands but is should replace the green band in the final output with the higher resolution band 4 image.

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

[djhoese]

Examples created by @kathys with Polar2Grid:

Before

After

[coveralls]

Coverage decreased (-0.004%) to 85.407% when pulling 9e91b27 on djhoese:bugfix-mersi-natural into a6ce4df on pytroll:master.

[codecov]

Codecov Report

Merging #939 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #939      +/-   ##
=========================================
- Coverage   85.41%   85.4%   -0.01%     
=========================================
  Files         172     172              
  Lines       26046   26046              
=========================================
- Hits        22246   22245       -1     
- Misses       3800    3801       +1

Impacted Files	Coverage Δ
satpy/readers/nucaps.py	93.71% <0%> (-0.53%)	⬇️

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[mraspaud]

LGTM