SCUBA2: no need to apply flatfield if given all data

Calibrator maps are made using all of the available data so the
current data should always be flatfielded before combing with the
existing flatfielded data. However, in the case where the summit
pipeline processes all of the data at once there is no need to
pre-flatfield the data before running the map-maker.
SCUBA2: don't warn for calibrators in the SUMMIT pipeline

The warning about getting into a running average with no Group file
does not apply to observations of calibrators processed with the
SUMMIT pipeline. The error was being triggered by the fact that the
pipeline waits until it has all the necessary data before making a
map, so the Group ends up with multiple members without having yet
created a Group file. In the SUMMIT pipeline all those Group members
contain flatfielded data to be given to makemap.