SCUBA2: default to deriving pixelscale, add support for different X,Y values

Let the default behaviour be to let the WCS primitive determine the
pixel scale automatically.

When printing the results to the screen, deal with the case where the
pixel scale is an array reference.
SCUBA2: tweak output to user
SCUBA2: no need to convert input system to lower case

...because all subsequent comparisons are made using upper case
SCUBA2: add support for separate X,Y pixel scales

Allow the pixel scale to be given as an array reference.
SCUBA2: add support for FPLANE coordinate system
SCUBA2: create and use a focal-plane reference image for mosaicking

The former dependence on knowledge of the instrument layout has been
removed now that the pipeline can create reference images in FPLANE
coordinates.