SCUBA2: limit box size to 15 pixels for fits to matched filtered images

Calibrator maps are made with 1" pixels so there's no point using a
fitarea that significnatly exceeds the FWHM. At 850 um this
corresponds to about 15 pixels, so use that as an upper limit.
SCUBA2: only check for two-component fit if requested

Testing has shown that two components fits are generally poorly
constrained and do not yield significantly better estimates of the
beam size and/or FCFs. Therefore, they are henceforth disabled by
default and only available on explicit setting of the FIT_GAUSSIAN
recipe parameter to 2. This limits the use of two component fits to
PICARD recipes as the main pipeline does not support this parameter.
SCUBA2: use a box of 15 pixels for all calibrator observations

A 25-pixel box is larger than necessary for calibrator observations so
reduced it to 15 pixels for all calibrations, not just pointings.

Also triple the box size for large planets since the maps are made
with 1-arcsec pixels.