SCUBA2: remove artifical (and unused) coadd mode

There's an easier way to tell if things are awry when making RUNNING
coadds. Rather than set an artificial coadd mode, check if the number
of members in the group is non-zero. If so, and there is no group file
then something weird has happened. Otherwise having no group file is
totally expected for the first pass through the primitive.
SCUBA2: ensure the Group contains the correct file if necessary

The "running" coadd is formed from the combination of the current
calibrated Frame image with the current Group image (which was formed
from the ... etc). In practice the coadd is tagged as the "reduced"
product so make sure the current file in the Group is that file. Of
course, this is not necessarily true in general but it is for the
current SCUBA-2 recipes.