add selection cut for Diboson event
As described in JIRA LHCBGAUSS-2167, we need a new selection cut for the Diboson events (ZZ,WW,WZ).
In the selection, the number of leptons can vary from 2 to 4, the number of bosons should be 2, while there is also an option for the mother particle of two bosons for Higgs -> ZZ events.
Edited by Gloria Corti