Add ProtoTrack structure
Currently we store this in output ROOT files outside of the eicd data model, as:
/// A proto track is a collection of hits identified by their indices. using ProtoTrack = std::vector<size_t>; /// Container of proto tracks. Each proto track is identified by its index. using ProtoTrackContainer = std::vector<ProtoTrack>;
Hmm, one-to-many relation...