SCUBA2: add group parameter, tag results

The new group parameter dictates which object to use in this
primitive.

The psf is stored in the current object and tagged with "psf", though
the obejct is returned to its input state on exit.
SCUBA2: PSF to whiten must be specified by a tag

If the primitive is whitening a PSF image, that image is now retrieved
from the current object using the tag "psf", provided it exists. There
is no alternative method.

The current object will contain either the whitened image or the
original image on input (if whitening a PSF).
SCUBA2: primitive can now retrieve PSF from psf or whitepsf tags

In order of preference, the primitive will now use a whitened PSF, or
a stored PSF or a PSF specified by recipe parameter. The first two are
retrieved via the appropriate tag.
SCUBA2: store and tag psf as well

The PSF is now stored and tagged with "psf" for later retrieval.

Also the cases where the group file does not exist or there are not
sufficient frames to create a group image are trapped.