Make the viirs_sdr reader return float32s

[mraspaud]

Various operations in the viirs_sdr reader were coercing data to float64 where it wasn't necessary.
This PR fixes this.

• Tests added
• Tests passed
• Passes flake8 satpy
• Fully documented

[djhoese]

Although I think this is a bigger problem that should be handled at the xarray level, this PR looks good to me.

[coveralls]

Coverage remained the same at 89.535% when pulling 8213c91 on mraspaud:fix-viirs-sdr-dtypes into d12bb7a on pytroll:master.

[codecov]

Codecov Report

Merging #1129 into master will not change coverage.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master    #1129   +/-   ##
=======================================
  Coverage   89.53%   89.53%           
=======================================
  Files         200      200           
  Lines       29356    29356           
=======================================
  Hits        26284    26284           
  Misses       3072     3072

Impacted Files	Coverage Δ
satpy/readers/viirs_sdr.py	85.95% <66.66%> (ø)	⬆️

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