Improve PI0DALITZ model.

Summary: Improve PI0DALITZ model to dynamically work out maximum probability to make it usuable also for eta --> llgamma decays. Model ETA2MUMUGAMMA is pure one-to-one copy of the PI0DALITZ and as such it is removed.

Test Plan: Will attach some test distributions

Reviewers: jback

Reviewed By: jback

Subscribers: #evtgen_admins

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D62