SCUBA2: a new primitive to remove the noisy edges from an image

This primitive will trim an image to a given map size (width & height
or radius). The default is to trim the map to the size given in the
FITS header.
PICARD: explicitly support rectangular/circular image crop methods

The addition of a default map radius requires the CROP_METHOD
parameter to explicitly support each of the two "spatial" crop
methods. This also means it cannot be assumed that the map radius
overrides the width and height.
SCUBA2: user can now supply an output tag/suffix

The TAG argument can now be given which is used to tag the output file
as well as define its output suffix.
PICARD: simplify by using new primitive

The main cropping/trimming is performed in _TRIM_MAP_EDGES_. This
simplifies the current primitive enormously.