Add DNB satellite angles (DNB_SENZ, DNB_SENA) to VIIRS SDR reader

[simonrp84]

Currently, the VIIRS SDR reader does not load the satellite zenith and azimuth angles from the geolocation file. This PR allows those datasets to be read if required (as is already done for I and M band data).

[coveralls]

Coverage increased (+0.02%) to 78.292% when pulling 0b6f83b on simonrp84:master into 2a49bf6 on pytroll:master.

[codecov]

Codecov Report

Merging #627 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #627      +/-   ##
==========================================
+ Coverage   78.27%   78.29%   +0.02%     
==========================================
  Files         136      136              
  Lines       19725    19725              
==========================================
+ Hits        15439    15443       +4     
+ Misses       4286     4282       -4

Impacted Files	Coverage Δ
satpy/readers/__init__.py	95.12% <0%> (+1.21%)	⬆️

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[djhoese]

Thanks!

[simonrp84]

Problem with the inclusion of two commits in one should be resolved, this should only include the VIIRS changes.