Add Edmonds-Karp maxflow algorithm
Reduce constant factor of maxflow algorithms
Merge ysitu's branch 'edmonds_karp' into refactor-flow.

New algorithm added, but still lacks the residual function and cannot be integrated in test_maxflow.py.
Add edmonds_karp_residual function, and edmonds_karp* to tests.
Expose compute_flow parameter in preflow_push_residual.

Also fixed a rendering issue in its docsting.