WIP: Decorators for Ghent heavy neutrino tree

Definitely to be improved... the tricky one are the lepton branches that "overlap" (there is one lepton index, but the muon or non-tau branches are shorter, _nMu and _nLight). I tried to represent that by providing the overall "_nL" collection, and also "el", "mu" and "tau" where the specific branches for the other flavours are taken out (as much as possible).

Next step: make a few plots and see if they make sense (I tested with 10 events so far)