New Bc -> Ks K psi model: LHCBGAUSS-1679

A new decay model for Bc -> K_S K J/psi, Bc -> K_S K psi(2S) decays was written; see the talk given at https://indico.cern.ch/event/797528/.

Instead of being put in a new class called EvtBcKsKPsi it is being included in the already existing EvtBcVHad model, extending its functionality.