Modernise EvtIdSet and other improvements
Summary:
Modernise and greatly simplify EvtIdSet implementation
Fixes in EvtPropSLPole to avoid unnecessary dynamic allocations
Other minor fixes and tidy-ups
Credit to Heather Ratcliffe and Chris Brady for providing
and/or inspiring several of these improvements
Test Plan: CI tests give identical results wrt master branch
Reviewers: jback, kreps, abudinen
Reviewed By: jback, abudinen
Subscribers: #evtgen_admins
Tags: #evtgen
Differential Revision: https://phab.hepforge.org/D98