Add di-Jpsi+tracks, etac->6pi, and update BPHY23

Following the successful reconstruction of X->Jpsi+etac->2mu+4pi: https://indico.cern.ch/event/1426493/contributions/6001516/attachments/2875627/5037113/New_channels_in_BPHY23.pdf We now add the channel X->Jpsi+etac->2mu+6pi. Tetraquark 3-body decay X6900 -> X6400 + 2pi is also possible, as well as the tri-charmonium production of 2Jpsi+etac, so we added di-Jpsi+tracks channels as well. One new c++ file (DiJpsiPlusTracksSingleVertex) is added for this purpose, and BPHY23 is updated.