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
Reviewed By: jback
Differential Revision: https://phab.hepforge.org/D62