Add support for Near-realtime VIIRS L1b data.

[simonrp84]

Currently, the viirs_l1b reader only supports archived VIIRS data from NASA. This PR adds support for the near realtime data, which has a slightly different filename structure.

[codecov]

Codecov Report

Merging #1420 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1420   +/-   ##
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  Coverage   90.58%   90.58%           
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  Files         236      236           
  Lines       33797    33797           
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  Hits        30615    30615           
  Misses       3182     3182           

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[coveralls]

Coverage increased (+0.02%) to 90.585% when pulling b91cc4e on simonrp84:viirs_l1b+nrt into 2d5bfdb on pytroll:master.

[djhoese]

I'm fine with this, but wondering if it could be more generic? Any idea if anyone uses something besides _NRT? Maybe a wildcard {processing_suffix} would be good? I know @mraspaud didn't like the idea of this for the ABI reader in a recent PR I made or at least not as an all-in-one file pattern.

Eh on second though maybe it is better to be explicit.

[djhoese]

Where does the NRT data come from? Is there a URL we could/should add to a docstring somewhere?

[simonrp84]

The NRT data comes from the same source as the 'normal' data that's already supported by this reader, they're both NASA products and are available from the same places (earthdata, f.ex).