PICARD: add ability to delete files after processing

The FCF calculation procedure generates a lot of intermediate
files. By default these are left on disk to enable closer inspection
of the results, but not all users will want that. Now, the KEEPFILES
recipe parameter may be given and all intermediates will be deleted if
set to 0.
PICARD: tag calibrated data as "calib" on entry to recipe
PICARD: rename and update POD

This recipe is designed to work on SCUBA-2 images only so its name
should reflect that.
PICARD: POD updates to expand on and/or clarify recipe behaviour
PICARD: update POD to document previously undocumented parameters

... and expand on/clarify recipe operation.