SCUBA2: find source position before creating cube

This primitive now coadds the images from each SMU position and
determines the offset of the source from the nominal 0,0 position
before creating the cube. This allows the shift to be applied to each
image before they are trimmed so the source should appear at the image
centre in the final cube.
SCUBA2: lower detection threshold in signal map to 3-sigma

This may seem odd but a S/N of 3 in the signal map corresponds to a
S/N of 5-6 in the matched-filtered map which is clearly good enough
for a detection and determining the peak position.
SCUBA2: determine all input parameters before proceeding

Allows for greater flexibility in inputs. After all there's no real
reason why a user giving a list of input files would not want the WCS
for them modified in place.
SCUBA2: always take out residual pointing offsets for calibrators

Except for pointing and focus observations.