Fix initialisations of constants
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
Maniphest Tasks: T101
Differential Revision: https://phab.hepforge.org/D27