point-neighbours x y z d procedure

  Takes a set of points and returns an undirected graph of those points which are near each other. The points are represented by three vectors, x, y and z, which must all be the same length, n. The ith point is at position (x(i),y(i),z(i)).

The value returned is a vector v of vectors of exact nonnegative integers. The vectors of integers v(i) represent the set of points near the ith point. If the point (x(i),y(i),z(i)) is within distance d of (x(j),y(j),z(j)), then i will appear in v(j) and j will appear in v(i). A point is never in its own set of neighbours.