fix PV fitter weigth calculation

small fix to the weight calculation in PV fitter. In the end, the difference was that for the numerator the chi2 used should be the one w.r.t to the initial seed position and not the one w.r.t to the updated PV position.
As side remark, the way the weights are calculated is a bit ad-hoc; I was thinking about changing it to make it more well-motivated, in the sense of moving more the original formula the calculation was initially was based