Add new features for BandQ double-charm hlt2 lines
According to the talk by @ibelyaev , events with charmonium + open charm particles reconstructed will be interesting for B&Q Run3 studies. This is still missing in doublecharm.py and lets try to add it.
And I notice that the current double-charm lines are just simple combinations of two charm builders, without any further selections. We may try adding a vertex cut, to make sure that the charm pairs are from the same PV. This selection might help to reduce BKG for Run3 where we have much more PVs than Run2.
Milestone: Bandwidth cost reduced to a reasonable value (30MB/s) in !1897 (merged) .
Next to do: