Skip already ingested images

Add a check in the image ingestion loop to see
if this file has already been ingested
(determined by checking if the filename and
region match an already ingested file).

We did already have a check in the method that
adds images to the database, but this is only
called  after the file has been opened and
processed, so uses unnecessary memory/time.

Note: The check on the region name is done by
looking in the archive location (to see if the
image is in the REGION subdirectory). It would
be more robust to check the region__region
field of the database entries, but that is
*significantly* slower.