Vertices information not propagated in the flow

[YifanC]

1. Do we want to have vertex time information? Currently it's not propagated in ndlar_flow. I imagine if we care about different interactions in the same spill/event, this information may be useful.
2. I have disabled the vertex information to be recorded if it's not from a neutrino generator, because the interaction information is inherited from mc_hdr. Perhaps, I should copy the vertices dataset in the flow output, even it's non neutrino simulation.