Update coefficients for FY-3B VIRR reflectance calibration

[djhoese]

In my continued effort to figure out why VIRR true colors look so terrible...

@kathys pointed me to some new coefficients for getting the reflectances out of the VIRR files. The images look much better.

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

[coveralls]

Coverage increased (+0.003%) to 84.648% when pulling 445dcff on djhoese:feature-virr-coeffs into ebede6d on pytroll:master.

[codecov]

Codecov Report

Merging #892 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #892      +/-   ##
==========================================
+ Coverage   84.64%   84.64%   +<.01%     
==========================================
  Files         171      171              
  Lines       25034    25046      +12     
==========================================
+ Hits        21190    21201      +11     
- Misses       3844     3845       +1

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_virr_l1b.py	97.39% <100%> (+0.19%)	⬆️
satpy/readers/virr_l1b.py	98.5% <100%> (+0.09%)	⬆️
satpy/scene.py	90.47% <0%> (-0.18%)	⬇️

---

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[codecov]

Codecov Report

Merging #892 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #892      +/-   ##
==========================================
+ Coverage   84.64%   84.64%   +<.01%     
==========================================
  Files         171      171              
  Lines       25034    25046      +12     
==========================================
+ Hits        21190    21201      +11     
- Misses       3844     3845       +1

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_virr_l1b.py	97.39% <100%> (+0.19%)	⬆️
satpy/readers/virr_l1b.py	98.5% <100%> (+0.09%)	⬆️
satpy/scene.py	90.47% <0%> (-0.18%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ebede6d...445dcff. Read the comment docs.