Rename MCParticle relations to particle to avoid clashes with type names

[tmadlener]

BEGINRELEASENOTES

• Rename MCParticle relations to particle in SimTrackerHit and SimPrimaryIonizationCluster to avoid clashes with the MCParticle typename.
  • Add getMCParticle and setMCParticle via ExtraCode to keep everything working.
  • This will break the reading of existing files via podio utilities

ENDRELEASENOTES

• Check if everything still builds with the ExtraCode

Fixes #247

[peremato]

Thomas I was expecting this PR to be part of the release you made yesterday.

[tmadlener]

Ah no. Yesterdays release was mainly to have a "last tag before things start breaking". All the PRs that are open now will either break existing code or files, so I wanted to keep them out of it.