Add enhancement for VIIRS flood reader

[katherinekolman]

This adds an enhancement for the VIIRS EDR Flood reader.

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

[coveralls]

Coverage increased (+0.04%) to 81.554% when pulling 70b64da on katherinekolman:viirs_edr_flood_reader into 81b3334 on pytroll:master.

[codecov]

Codecov Report

Merging #532 into master will increase coverage by 0.03%.
The diff coverage is 94.82%.

@@            Coverage Diff             @@
##           master     #532      +/-   ##
==========================================
+ Coverage   81.51%   81.55%   +0.03%     
==========================================
  Files         154      157       +3     
  Lines       22570    22623      +53     
==========================================
+ Hits        18398    18450      +52     
- Misses       4172     4173       +1

Impacted Files	Coverage Δ
satpy/tests/enhancement_tests/test_enhancements.py	98.55% <ø> (ø)
satpy/enhancements/viirs.py	100% <100%> (ø)
satpy/tests/enhancement_tests/test_viirs.py	100% <100%> (ø)
satpy/tests/__init__.py	93.1% <100%> (ø)	⬆️
satpy/readers/viirs_edr_flood.py	90.24% <50%> (ø)	⬆️
satpy/tests/enhancement_tests/__init__.py	90% <90%> (ø)

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[djhoese]

@mraspaud @pnuu I accidentally requested your review on this and github won't let me remove it. I meant to do it on #580.