Add a OneToMany relation from TrackerHit to RecIonizationCluster

[BrieucF]

With the objects added in PR#179 for cluster counting algorithms, the only way to get the number of ionizations associated to a given TrackerHit is to do a matching by cellID which is quite inefficient. This PR proposes to add a link between TrackerHit and RecIonizationCluster to simplify the derivation of dN/dx.

BEGINRELEASENOTES

• Add a OneToMany relation from TrackerHit to RecIonizationCluster to allow us deriving the number of clusters associated to a TrackerHit without having to do a matching by cellID

ENDRELEASENOTES

[BrieucF]

I would update the line numbers in the README.md once we agree on the way to go, before merging. By the way, isn't there a script somewhere that does it automatically?

[tmadlener]

By the way, isn't there a script somewhere that does it automatically?

No, but there really should be ;)

[andresailer]

Closing (at least temporarily) while DriftChamber Tracker hit is under development by @BrieucF