Also renamed files in the flow package so modules and functions have
different names; we obey the informal module-name-must-be-unique rule.
Updated tests and docstrings. The later needs a bit more work to clearly
explain that flow functions have to be explicitly imported from the
flow package.

This solution is not very nice but works.

This commit introduces backwards incompatible changes to the interface
of flow algorithms:

1. Split maximum_flow into maximum_flow (returns a tuple flow_value, flow_dict)
and maximum_flow_value (returns only the value).

2. Split minimum_cut into minimum_cut (returns a tuple cut_value, partition)
and minimum_cut_value (returns only the value).

3. Remove the short versions max_flow and min_cut.

4. Remove flord_fulkerson_flow and ford_fulkerson_and_auxiliary

5. Remove the parameter `legacy` from ford_fulkerson. It now returns a
residual network as all other flow algorithms. I also tried to remove it
but doing so requires quite a few changes in mincost.py because the code,
tests, and the docstrings still relay on the its obsolete convention
for flow_dict.

Check that kwargs is empty if flow_func is None, if not raise NetworkXError. This way users will nor rely on the default_flow_func.

Also some other small docstring fixes.

The criterion for determining minimum cuts is also clarified.