Update 'viirs_edr_active_fires' reader to read newest algorithm output

[djhoese]

This was first discussed on slack and brought up by @ColinDuff. The reader originally written by @rdaruwala was based on developer/beta versions of the products created using the CSPP Active Fires software. These files started with AFEDR, but the software has now been updated to have an AFIMG and AFMOD prefix for imagery (I band) and moderate (M band) resolution output. Some examples can be seen from the SSEC direct broadcast FTP location:

ftp://ftp.ssec.wisc.edu/pub/eosdb/npp/viirs/2019_04_14_104_2042/edr/

From what I've heard from @kathys, the AFIMG output is different than the AFMOD files so some more complex changes may be needed to fix this.

The file patterns for the current reader should be replaced with the new filename scheme. I'd also like to see if the files that can be ordered from NOAA CLASS can be read with this same reader, see https://data.nodc.noaa.gov/cgi-bin/iso?id=gov.noaa.ncdc:C00971. If it isn't just a simple file pattern addition then I'd consider support for CLASS files as out of scope for this issue. I've put some files here which will only last for 7 days.

I've asked @rdaruwala to work on this this week.