SCUBA2: combine and use multiple images if present

This patch fixes a long-standing bug where multiple images for a given
SMU position were not actually being used in the focus cube.

Also increase the trimmed map size to 120" to allow for the source
being off-centre by a large amount (if a pointing was not done before
the focus).
SCUAB2: don't bother processing data if the focus fit has already been done

If there are multiple subscans per SMU position, the focus cube may be
created before the last one for the final SMU position has been
observed. Should the pipeline be fast enough to end up looking at
these data then there is not point processing them or re-calculating
the best fit focus position.