Fix viirs sdr reader to allow ivcdb files in the sdr directory

[adybbroe]

CSPP (and IPOPP) may output Dual Gain calibration files with the prefix IVCDB. As of now, when having these files in the directory together with the other SDR files, satpy fails with a KeyError

CSPP version 3.2 will output these files on default.

• Closes #xxxx
• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff
• Fully documented

[djhoese]

My initial reaction is that users should be providing files that can't be read by the reader. However, it is also the readers fault for matching files that is doesn't know how to read. However (again), the VIIRS SDR reader is a pretty special case.

@mraspaud can this be updated to not have a KeyError if it doesn't understand one of the file types? I'm ok adding what @adybbroe has so far, but can we make it so things don't completely die?

[mraspaud]

Yes, I was wondering about it also, but I don't think we have a way of gracefully failing the filehandler init...

[djhoese]

Ah ok, sounds good.

[codecov]

Codecov Report

Merging #731 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #731   +/-   ##
=======================================
  Coverage   80.02%   80.02%           
=======================================
  Files         147      147           
  Lines       21498    21498           
=======================================
  Hits        17203    17203           
  Misses       4295     4295

Impacted Files	Coverage Δ
satpy/readers/viirs_sdr.py	85.02% <ø> (ø)	⬆️

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[coveralls]

Coverage remained the same at 80.021% when pulling d6ab8b4 on adybbroe:fix-viirs-sdr-reader-ivcdb into 1849b17 on pytroll:master.