Add new ShadowParticle Attribute for linking between GenEvents
The new ShadowParticle HepMC3::Attribute class is intended to be used to
link GenParticle objects between GenEvent objects.
Currently not used, but is intended to be used as part of the refactoring of
the quasi-stable particle simulation code in !63059 (merged).
The purpose of that
development is to allow ATLAS to migrate to using GenParticle::id() to link
EDM objects to the HepMC truth, rather than the barcode Attribute (using the barcode
Attribute is around 10% slower for some production steps than using a GenParticle member variable, as can be
seen when comparing performance between master and master--HepMC2, where barcode was a GenParticle member variable).
Edited by John Derek Chapman