allow both flatfield and polynomial re vignetting correction

Make nona accept flatfield\whitefield/dustmap file and vignetting polynomial coefficients simultaneously.

In hugin 0.6 there was a choice of flatfield file
and polynomial for vignetting correction. I can
imagine that nona currently only allows one of
the two methods to be used.

May I suggest that both flatfield and polynomial
could be used at the same time. The flatfield would
usually be a dustmap only, and the polynomial would care
for the vignetting correction as currently is the
case for hugin version 0.7.0 minus epsilon.

Klaus