Chordal graph package update

I've fixed some bugs and added chordal-usage.nb (HTML) notebook with examples on how to do the following things with the chordal graph package:

- checking if the graph is chordal
- making graph chordal by adding fill edges using minimum fill heuristic
- getting perfect elimination ordering of a chordal graph
- arranging chordal matrix in perfect order
- arranging general sparse matrix in order that minimizes extra fill added by cholesky decomposition
- getting maximal cliques of a chordal graph
- getting clique tree structure of a chordal graph
- generating a chordal graph with given tree width and given tree structure

Description of methods is in Blair's excellent "An introduction to chordal graphs and clique trees" monograph


Chordal graphs release2