Introduced function to set seed.

Summary:
Introduced function setSeed for random engines. Related with !26.

Set seed during event generation in testing.

Specify functions that are overriden in EvtMTRandomEngine class.

This is related with the general thread-safety modifications: T168.

Test Plan: All tests run flawlessly. Outputs will change within statistical uncertainties as we now set the seed by hand for each event.

Reviewers: tlatham, kreps, jback

Reviewed By: jback

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D121