Implement a small cviirs speedup

[mraspaud]

This is a minor improvement to the cviirs reader, which helps in case of slow IO

• Tests passed
• Passes flake8 satpy

[coveralls]

Coverage decreased (-0.002%) to 87.201% when pulling f474e24 on mraspaud:feature-small-cviirs-speedup into e8f3518 on pytroll:master.

[codecov]

Codecov Report

Merging #1012 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #1012      +/-   ##
=========================================
- Coverage    87.2%   87.2%   -0.01%     
=========================================
  Files         181     181              
  Lines       27890   27893       +3     
=========================================
+ Hits        24321   24323       +2     
- Misses       3569    3570       +1

Impacted Files	Coverage Δ
satpy/readers/viirs_compact.py	64.98% <100%> (+0.41%)	⬆️
satpy/scene.py	90.35% <0%> (-0.18%)	⬇️

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