SCUBA2: check that the Frame has a single file

The check for a single file in the Frame is to allow PICARD to use
this primitive. Note that it must be the Frame that is queried not the
current object as determined by the GROUP parameter as the presence of
a group file will cause this block to entered each time with a high
probability of failure since the logic expects some header entries to
*not* be in the subheaders.