fastjet/internal/delaunay: add function SecondNearestNeighbor

Created by: Bastiantheone

Add the function SecondNearestNeighbor to handle when a jet's nearest neighbor is its own mirror point.
Mirror points can be identified in runNlnN, but then the nearest neighbor that is not the mirror points needs to be found. That is when SecondNearestNeighbor comes to use.