adding boilerplate for graph-matrix
clean up matrix methods in graph.lisp

1. Remove to-identity-matrix because it will be just as easy and
   probably more readable for users to do something like

       (make-identity-matrix :length (length (nodes graph)))

2. Use nil for the value of missing edges in a value matrix

3. Renamed from-identity-matrix to the more accurate from-value-matrix.
adjacency-matrix doesn't need to know edge values
generate a gsll matrix from a graph