PICARD: replace RA,DEC parameters with generic X,Y for registering images

To allow images to be registered in Galactic coordinates, the
REGISTER_RA and REGISTER_DEC recipe parameters have been replaced by
corresponding X and Y parameters.

A naive test is used on the X,Y values to determine the coordinate
system: if the X and Y values contain a colon, then it is assumed the
coordinates are J2000, otherwise Galactic.

Note that now the X,Y position is stored in the Frame uhdr as an array
reference, not as separate entries.
PICARD: document change to X,Y from RA,DEC
PICARD: RA,DEC replaced by single array reference

The separate REGISTER_RA and REGISTER_DEC recipe parameters have been
replaced by an array reference for the X,Y position (REGISTER_XY).
PICARD: delete intermediate files created during processing
SCUBA2: add support for Galactic coordinates

Images can now be aligned in Galactic coordinates if given a suitable
reference position. Note that the reference position must still be in
the same coordinate frame as the input images. A future upgrade may
relax this constraint.