Allow shared hits in track_building eval and fix typo

• Allow hits to be shared by multiple tracks or multiple particles in the eval stage of track_building
• Also fix the typo edge_track_labels -> hit_track_labels
• Remove a few redundant lines

More explanation:

By nature hits can be shared by multiple particles. In the current code, if a hit is shared by multiple particles, we randomly assign the shared hit to one of the particles it corresponds to. This will result in inaccurate matching of the track candidates.

In addition, I enable the possibility of a hit to be shared by multiple track candidates (in the eval stage). Our current track building methods don't reuse hits with different tracks, but in case we have such an algorithm that reuses hits in the future (e.g. the junction resolve I presented earlier), we can correctly handle it in the eval stage.

I have tested the changes with the CCandWalk and FastWalkthrough and saw very little change in the track performance computed in the eval stage of track building in acorn (i.e. technical efficiency, etc.) (which is expected because the fraction of shared hits in our sample is very small).