Get PNG product name from header and merge vectors

It is no longer the case that each group's main product will
be called "reduced", so it is necessary to determine the
product name for the preview images from their headers rather
than assuming that it will always be "reduced".

Also updated the PNG image merging subroutine to deal
with vector plots, which should be composited with
the representative images.
Allow multiple files per product name

There can now be multiple files with the same product identifier,
so we need to detect this situation and disambiguate the files
by re-appending their original suffices.
Update UKIRT product suffix patterns

There should apparently no longer be no-suffix products.  Polarimetry
products have been renamed as they will now go into one plane in
CAOM-2.