SCUBA2: ensure position really is undef as described in the documentation

The primitive was storing a reference to an empty array, which is not
the same as undef. Now the position is only stored if an x,y position
was determined successfully.
SCUBA2: increase fitting area to 19 pixels

Increasing the fitting area slightly helps obtain better fits for some
weaker sources.
SCUBA2: also use matched-filtered map to assess whether a peak is detected

For a genuine source detected at low signal-to-noise, applying the
matched filter should more or less double that signal-to-noise
ratio. Therefore a way to test if an image is blank is to check to see
that the peak S/N in the matched filtered map is indeed higher than in
the signal map. Blank maps have a largely unchanged peak S/N.
SCUBA2: return full statistics to caller if requested

The new argument STATS is a flag to indicate whether or not the
statistics of hte SNR image should be calculated and returned to the
caller. Default is true.

Also the region over which the stats are calculated is now an input
argument.