updated fast background to be really linear (a1 and A2 instead of Ax and
A*(1-x))
updates in the levenberg marquardt fitter for linear fitting, but still
bugs
still problemns with linear fitting in levenberg fitter
levenberg fitter improved for linear: works for a line on simple data,
but not on a real spectrum...probably problem in scaling of Yprime
linear fit works, but not with weighing with variance
there seem to be remaining issues with convergence speed for nonlinear
as well (possibly also related to weighing)