Improvements in EvtId and EvtPDL

Summary:
Minor modernisation of EvtId.

Improve Boolean functions in EvtId class to return Boolean instead of integer.

Converted EvtPDL to thread-local singleton.

Propagated Changes in EvtPDL to EvtGen initialisation.

This is related with !31 and T168.

Test Plan: Tests give identical outputs with respect to master.

Reviewers: tlatham, kreps, jback

Reviewed By: tlatham, jback

Differential Revision: https://phab.hepforge.org/D128