SCUBA2: remove unused STORE parameter

In those cases where STORE was specified, it was always false which
renders it pointless.
SCUBA2: remove deprecated STORE parameter
SCUBA2: remove unused refimage method and attribute

This method and associated attribute was added a long time ago with
the intention of implementing something useful. In practice, the
calibration object isn't the best place for storing a reference image
as there is the risk that it is picked up and used by different
Groups, a scenario that is unlikely to end well.
SCUBA2: add refimage method for storing/retrieving reference images

A reference image is useful for defining coordinate systems when
mosaicking or coadding images. This method stores the name of files
that may be used as references and allows separate files to be stored
for celestial or focal-plane mosaics as both types are needed in the
pipeline. The Group uhdr is used rather than add extra object attributes.
SCUBA2: use new Group refimage method for reference image
SCUBA2: always create reference for focal-plane mosaics

The QL pipeline needs all the help it can get to keep processing time
to a minimum, and creating a reference image each subscan
unnecessarily adds a second or two. Not only that, it turns out that
the noise/NEP mosaics weren't even using a valid reference, instead
always using the first file in the Frame. The whole point of having a
reference is to ensure that the mosaics are always oriented the
correct way, which is not guaranteed when using one of the subarray
noise images as a reference.

Therefore always create a reference (if one does not already exist) if
the mosaic is in focal-plane coordinates even for science sequences so
the QL will be able to reuse a suitable reference.
SCUBA2: store the current mosaic as the reference if required

When creating the focal-plane mosaics in the QL and SUMMIT pipelines,
the reference image needs to persist over multiple sequences. Rather
than leave a series of dedicated reference images on disk, it is
preferable to use a persistent output file such as the noise/NEP
mosaic. Therefore, store the current mosaic as the reference - unless
a reference has already been stored. This is required to avoid the NEP
mosaics being stored as the references as they are deleted at the end
of the recipe.