Circumvent non-storing of the gen particles by the framework:
add new collection tt_genParticles of objects of type "GenParticle", storing the gen_p4 and pdg id.
indices previously pointing to the framework's collection point to this new collection. They now default to -1 instead of 0, since 0 is a valid entry in the vector (and some indices might be non-defined)
Circumvent non-storing of the gen particles by the framework: