Add LN2_NEIGHBOR_MATH

[ofgulban]

A new program that can read the text output from LN2_NEIGHBORS might be useful for cluster labeled voxels with first order neighborhood information (I think of them as conveniently processing "connected point clouds data). E.g. processing the clusters/labels coming from LN2_COLUMNS or LN2_HEXBIN.

For the initial implementation steps, I can compute:

1. Mean
2. Variability
3. Correlation