Improve HepMCParticleLinkCnv unit tests

The discussions in ATLASRECTS-5459 made it clear that the unit testing of HepMcParticleLinkCnv_p1 and HepMcParticleLinkCnv_p2 needed to be extended. This merge request adds checks for links to GenParticle objects which are not part of the first GenEvent in the McEventCollection. It also adds a check to ensure that links are correct when the position of a GenEvent in the McEventCollection rather than its event number is used.

There is a second commit which makes HepMcParticleLinkCnv_p1 more robust in order to facilitate any possible future tests of switching back to using HepMcParticleLink_p1 as the persistent format written out by master.