PICARD: add KEEP and VERBOSE as arguments

KEEP is passed to the file deletion primitive as is, unless modified
according to the particular recipe.
PICARD: document default PSF, delete intermediate files

Add documentation to describe how the default PSF is based on a model
of the JCMT beam.

Intermediate files are now deleted, keeping _mf and _psf by
default. Setting KEEPFILES=0 will only delete the _psf: the
matched-filtered map is always kept (which seems like a reasonable
assumption...).