Add option to bypass fitFluxReference for very short exposures

There are PIs interested in very short exposures (<1min, even 10sec). FLUXSTDs prepared by the observatory are too faint for such exposures. We can ask the PIs to prepare FLUXSTDs by themselves, but I think it would also useful to have a pipeline option to use 'fiducial' fluxCal vector (which we can generate by averaging fluxCal vectors form many visits taken under reasonable conditions) and skip fitFluxReference. We should also skip the focal plane fitting in fitFluxCal and simply apply the 'fiducial' fluxCal vector to all objects. The fluxCal accuracy will be poor, but it should at least give the 1st-order correction. People interested in short exposures are stellar people and most of them do not use the continuum information in any case (so the poor fluxCal is not a major problem).

This ticket is not a high priority, but I am filing it so that I do not forget.