add Xib2Xic3pi, Xic2Xi3pi decfiles

Add decfiles for

• \Xi_b^0 \to \pi^- \pi^+ \pi^- \Xi_c^+(\to \Xi^0 \pi^+ \pi^- \pi^+ )
• \Xi_b^- \to \pi^- \pi^+ \pi^- \Xi_c^0(\to \Xi^- \pi^+ \pi^- \pi^+ )

We want the \Xi / \bar{\Xi} to understand the expected background of \bar{\Xi} when looking for anti sexaquark (uuddss) annihilation in detector material