general: set ORACUT in the hdr if not set, using DATE-OBS

In some cases the ORACUT entry in the Frame or Group hdr may be
undefined, because the UTDATE FITS header is missing from the
file. This happens, for example, when SCUBA-2 images from different UT
dates are coadded, which causes calibration attempts to fail. Without
further information, the simplest option is to set a dummy ORACUT
using the UT date of the DATE-OBS which should always be defined.
PICARD: print error if no FCF could be determined

In most cases this will be caused by a missing ORACUT entry in the
hdr. Set a value from DATE-OBS and re-try to obtain a suitable FCF.
SCUBA2: trap undefined FCF, set ORACUT and try again

Calibration of coadds/mosaics made from data from different UT dates
will fail because the ORACUT header is not defined. Furthermore, the
FCF returned by the calibration object in this case is 0 leading to
empty (zero-filled) maps.

Now issue an error that ORACUT was not defined and inform user that
calibrating coadds is not recommended. However, proceed with setting a
suitable ORACUT from the DATE-OBS and try again to retrieve an
FCF. This time it should succeed and the calibration can proceed.

Most of the change in this commit just comes from wrapping the
calibration step in a check for a defined FCF, which issues an error
if none could be determined.