Add testcases to test_maxflow_large_graph

These networks are generated using the same generators and parameters as
in the 1st DIMACS Implementation Challenge. But they are not the same as
the actual DIMACS instances due to different random numbers being used.
Clean up heuristics logic in preflow-push maxflow algorithm

This ensures that during phase 1, if a node is found to be disconnected
from the sink in the residual network, it will not be further processed
until phase 2.
Minor readability changes in preflow-push maxflow algorithm
Allow retrieving residual network in preflow-push maxflow algorithm

The interface is intended to be internal and for use in the minimum cut
algorithm.
Fix examples in docstrings of preflow-push maxflow algorithm
Make frequency mean frequency for preflow-push global relabeling