fitPfsFluxReference.py should not give up when multiple broadband fluxes are unavailable

fitPfsFluxReference.py usually requires two or more broadband fluxes for a FLUXSTD spectrum to be fitted---the radial velocity must be measured before the FLUXSTD spectrum is compared with synthetic spectra, a goodish-fit synthetic spectrum must be chosen for the radial velocity to be measured, and a number of broadband fluxes must be given to get the goodish-fit synthetic spectrum.

The test inputs, however, have only one broadband flux per FLUXSTD spectrum.

To make tests easier, fitPfsFluxReference.py should go on and use a typical synthetic spectrum to measure the radial velocity if only a single broadband flux is available.