SCUBA2: rewrite to use more robust method using matched filter

This primitive now uses the method first included in _FIND_MAP_PEAK_
where the matched filter is applied to the image to search for a
peak, which will show up as a more significant maximum if present.

The criteria for a peak are now that the peak S/N in the signal map
must exceed 3, the peak S/N in the matched-filtered map must exceed 5
and the matched-filtered peak S/N must be greater than the signal S/N.

If all three are satisfied, the position of the peak S/N is returned
to the caller in the primitive hash.

Note, however, that if given a map that has already been processed
with the matched filter, the only criteria used is that the peak must
exceed 5-sigma. This assumes that if true, the original map must have
had a minimum 3-sigma peak.

If a fit to the source fails, the value at the peak position and its
associated uncertainty are returned.
SCUBA2: factor out image assessment to a different primitive

The assessment of whether or not the input image contained a peak is
now handled by the updated _ASSESS_IMAGE_.

The code has been tidied up to remove unused or no-longer-used
variables and statements, and the POD updated accordingly.
SCUBA2: update to reflect changes in image assessment primitive

_ASSESS_IMAGE_ now returns PEAK in the primitive hash, not POINT, and
there is no longer any distinction made between point-like and
extended sources: the image either has a peak of some kind or it
doesn't.
SCUBA2: use only flatfielded data in the SUMMIT pipeline

In the SUMMIT pipeline a map should only be made using files with
flatfielded data, if flatfielded data exist. In this case fastflat and
noise data should not be included in the list given to makemap. (Note
that if no flatfielded files exist, then the map is being made from
all of the raw data taken thus far and it's OK to include the
fastflats.)