ISF_Event: HepMC3 fixes for HepMcParticleLink.

Scott Snyderrequested to merge
ssnyder/athena:mclink.ISF_Event-20220325 into master

For HepMC3, HepMcParticleLink should yield a ConstGenParticlePtr rather than a bare pointer.

Fix bug with use of shared_ptr that would lead to a double-deletion.

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