Very High Level Algorithm for Implementing Random Walker

1. Input image (and convert to "double precision image"???)
2. Build an empty graph (lattice; all edges weight=1) using dimensions of image (in pixels)
3. Generate edge weights
4. Generate Laplacian matrix
5. Determine locations of labels
6. Set up Dirichlet boundaries
7. Solve Random Walker algorithm by solving the combinatorial Dirichlet problem
8. Generate mask