Add EvtD0ToKspipi DP model.

Summary: Add EvtD0ToKspipi DP model, courtesy of Camille Normand (LHCb), based on D119.

Test Plan: Added two JSON test files, one with the D0 mode only and another including the parent B decay.

Reviewers: abudinen, kreps, tlatham

Reviewed By: tlatham

Subscribers: #evtgen_admins

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D124