Add EvtEtaLLPiPi model.

Summary: Add EvtEtaLLPiPi model for eta' -> l l pi pi decays (l = e or mu), for https://phab.hepforge.org/T109.

Test Plan: Model has been tested using testDecayModel (in the jsonTest branch).

Reviewers: kreps, tlatham

Reviewed By: tlatham

Tags: #evtgen

Maniphest Tasks: T109

Differential Revision: https://phab.hepforge.org/D34