SCUBA2: scale the default box size by the actual pixel size

The assessment of whether a source is point-like is done in pixel
space. An image of a point source made with 1" pixels at 850 um does
not pass the point-source test on account of the source covering many
more pixels than assumed. Thus the default box size should take into
account the pixel scale such that maps with small pixels use a larger
default box size.

This scheme should still work for large pixels, though it has not been
tested.

It is still assumed that a user-provided value for box is correct.
SCUBA2: increase the default map size to 75 arcsec

Due to a strange interaction between this default map size and the
default search area for _CENTROID_IMAGE_, certain maps fail the
criteria for proceeding with a centroid estimate, even if the data are
perfectly good. The simplest solution is to increase the default map
size to something not so easily related to the default centroid box.