Add a new decay file called Xib-_XicDs,pKKpi,KKpi=DecProdCut.dec

Add a Decfile for the decay Xib- -> Xic0 Ds- with Xic0 -> p K K pi and Ds -> K K pi, decay products in acceptance:

• Xib- -> Xic0(->p+ K- K- pi+) Ds-(->K- K- pi+)