Discrepancies on the expected Branching Ratios for HNL decays in MeVPrtl module

[LuisPelegrina]

HNL decay cross sections are wrongly calculated in MeVPrt module, leading to an incorrect estimation of the expected flux of HNL. HNL decays to two leptons and a neutrino are overestimated, leading to an underestimation of the number of events associated with other decays. The cros section of the decay leading to two leptons of different flavor is also not included in the calculation of the decay weight.