Fix initialisations of constants

Summary:
Initialisation of constants was not done when generating events,
which resulted in infinite loop as kinematics could not be generated.

Test Plan: Checked that after fix, code does not get stuck anymore.

Reviewers: jback, tlatham

Reviewed By: jback, tlatham

Tags: #evtgen

Maniphest Tasks: T101

Differential Revision: https://phab.hepforge.org/D27