Given a pattern of points on a linear network, this function computes the
nearest neighbour distance for each point (i.e. the distance
from each point to the nearest other point), measuring
distance by the shortest path in the network.
If method="C"
the distances are computed using
code in the C language. If method="interpreted"
then the
computation is performed using interpreted R code. The R code is
much slower, but is provided for checking purposes.