SCUBA2: amend comment to not spread misinformation...
PICARD: use current Frame and tags, plus option to combine files in Frm

A fairly substantial rewrite to improve flexibility. The new version
can now combine all images in the current Frame, and deals with S/N
images correctly (which lack EXP_TIME etc components).

The old code that pretended to align the EXP_TIME and other components
has been removed, and the relevant primitives do their own assessment
of whether these components exist.

All processing takes place on files stored in the current Frame
(rather than temporary Frames), which is returned to its input state
at the end of processing.

The Group product is defined as "newimage".