Derive EvtFlatSqDalitz from EvtDecayIncoherent as we directly provide final kinematics

Summary: Fix base class of EvtFlatSqDalitz, which was overlooked when rewriting model.

Test Plan: Ran test checking square Dalitz plot to be flat and it is as expected.

Reviewers: jback, tlatham

Reviewed By: jback, tlatham

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D81