SCUBA2: factor out two-component check into new primitive

The check for whether a two-component fit can be attempted must now be
carried out before calling the fitting primitive, which will now fit
as many components as requested through the parameter TWOCOMP.
SCUBA2: check whether a two-component fit can be carried out

Do this check before calling the fitting primitive, which allows a
dynamic fitarea to be determined when determining the beam size. A
two-component fit uses a larger fitting area to ensure the secondary
beam is included.