SCUBA2: store signal map and psf separately and tag each

This allows downstream primitives to select the relevant data based on
the tag, rather than having to know the order in which the files were
stored.
SCUBA2: allow tagged files to be coadded

A new parameter MOSTAG allows files tagged with this value to be
selected and coadded. The default behaviour is to use all of the files
from the good Frame objects (as before).

Also store the number of files in the coadd in the uhdr NMOSFILES.
SCUBA2: combine files by tag rather than index

The primitive now selects files tagged with "newimage" and "mappsf"
for coadding rather than relying on knowledge of the index into the
list of files in the Frame objects.
SCUBA2: generalize name of coadding primitive

As with the now-called _MAKEMAP_FRAME_PSF_, a name change was in order
to reflect the use of this primitive beyond the CLS.