Added shower startpoint correction post-processor and reco particle attributes

[dkoh0207]

The shower startpoint correction post-processor (showerstart_correction_processor) re-computes the shower start to a point in shower closest to the estimated reco vertex.

Also added two new attributes to RecoParticle:

1. vertex_distance: the closest distance between all points of a RecoParticle object and RecoInteraction.vertex.
2. shower_split_angle: the estimate of the angular separation between the "two major directions". Used for rejecting $\pi^0 -> \gamma \gamma$ decays in $\nu_e$ selection. The shower_split_angle is computed using the following procedure:
   • Compute the displacement from the vertex to all shower points.
   • Normalize the displacement vectors to the unit sphere. This gives a distribution of "directions" on the unit sphere, with the sphere center as the vertex.
   • Cluster the unit vectors using DBSCAN with cosine distance.
   • The shower_split_angle is defined as the maximum angle between the mean direction vectors of each DBSCAN cluster on the unit sphere.