PICARD: delete intermediate files
SCUBA2: only insert a new FITS header item if one does not already exist

If the FCF FITS header already exists, overwrite with the new value.
PICARD: print warnings only under certain circumstances

Add a new warning that input data are already calibrated in the
desired output units.

Only warn that no FCF was derived for occasions when that might be
useful to know; i.e., when the picard_results hash is populated.
PICARD: remove the FCF FITS header item if present

Undoing the calibration means removing all traces of the FCF,
including in the FITS header.