Fix ratio sharpening compositors when the ratio is negative

[djhoese]

This fixes a logic bug and cleans up the code for the RatioSharpenedRGB compositor class. This fixes the bright cyan pixels on the edges of clouds in things like the ABI true color RGB. I still need to add tests.

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

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@12298a9). Click here to learn what that means.
The diff coverage is 99%.

@@            Coverage Diff            @@
##             master     #548   +/-   ##
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  Coverage          ?   75.97%           
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  Files             ?      136           
  Lines             ?    18807           
  Branches          ?        0           
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  Hits              ?    14288           
  Misses            ?     4519           
  Partials          ?        0

Impacted Files	Coverage Δ
satpy/tests/compositor_tests/__init__.py	98.95% <100%> (ø)
satpy/composites/__init__.py	59.43% <96.55%> (ø)

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