Adding Backwards Tracks variable in Lumi PV tupling

Modifies algorithm in RecAlgs/LumiPVs_nobeamline.cpp to add a new variable PV_nTrBwto the tuple, which counts the number of VELO backwards tracks associated with each primary vertex, needed for BGI studies. This updates !4022 (merged) that didn't include this variable.

To be merged in the end with !4298 from @nmchugh for vertex splitting. We proceed with two different MRs to go forward quickly with the ghost charge analysis that requires this new variable.

cc Lumi convenors @efranzos @vazhovko