WFCAM: Trim number of decimal places in error message.

Two decimal places are quite sufficient when the mean value is
too low.
ifu: Enable creation of group representative white-light image.

There was a further use of the FLUX_CALIBRATED internal header which
could test the Grp instead of the Frm object, where this
boolean header is retained.  This was missed in 448febd4fec04.