PICARD: add support for FAKEMAP_OFFSET

It is useful to be able to take notice of the FAKEMAP_OFFSET recipe
parameter when creating an artificial PSF in the jack-knife recipe to
ensure the PSF ends up at the image centre.
SCUBA2: derive pixel offsets if not already determined

This condition is added for PICARD which does not know about the
OFFSET_PIXELS uhdr, and instead just needs to calculate the values
directly.
SCUBA2: don't reuse variable names...

Two variables called "thresh" = ample scope for confusion. Use a more
meaningful name for the thresholded file instead.
SCUBA2: use the given PSF name if present

The correct conditional check is to see if the PSF has been given by
the user and use that, otherwise fall back on the one created here,
which will only happen in the pipeline. Calling this primitive from
PICARD requires the user to provide their own PSF.
SCUBA2: add option to produce a normalized PSF

This option overrides all other input parameters. The output PSF will
have an amplitude of unity.