Convert a list of mesh edges to a graph or matrix.
[A,xy] = UNMESH(M)
Each row of M is a mesh edge in two dimensions, [x1 y1 x2 y2].
A is the Laplacian matrix of the mesh (the symmetric adjacency matrix
with -1 for edges and degrees on diagonal).
Each row of xy is a coordinate [x y] of a mesh point.
See also MPLOT, GPLOT , SUBMESH.