TTreeReader in ROOT6

[kreczko]

http://indico.cern.ch/event/304944/session/4/contribution/411/material/slides/1.pdf [slide 7]

Might be useful to simplify our code (post EA)

[kreczko]

From https://root.cern.ch/root/html604/notes/release-notes.html#patch-releases
TTreeReaderArray now supports vector<builtin-type>.
All the pieces are now together