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