Implement max flow and min cut

[simonbowly]

Description

Continuation of #51.

For a new Mod contribution, please include the issue number for the Mod proposal and check through the list below.

Checklist

• Implementation:
  • Implementation of the Mod in the gurobi_optimods installable package
  • Tests for the Mod implementation in tests/
  • Docstrings for public API, correctly linked using sphinx-autodoc
• Documentation page:
  • Background and problem specification
  • Example of the input data format (use gurobi_optimods.datasets for loading data)
  • Runnable code example
  • Presentation of solutions
  • Included in the mod gallery and toctree

Have a nice day!

[simonbowly]

@torressa looking good! I made some adjustments to the docstrings and doctests to align with changes we made to the rest of the mods. Please see my comments for some further changes I think are needed here.

[torressa]

@simonbowly I think I've addressed most comments and may have messed up the commits again.

[CLAassistant]

All committers have signed the CLA.

[simonbowly]

@torressa sorry for the delay on this one. I have a couple more suggestions:

• Can the formulation code be re-used between max-flow and min-cut? The model is exactly the same, right?
• In the docs, I think the solution section would be easier to follow if it were split up. At least the code blocks showing the max-flow/min-cost results should be separated for readability, and it may help to separate the figures showing the max flow and min-cut results.

[torressa]

Can the formulation code be re-used between max-flow and min-cut? The model is exactly the same, right?

It is, yeah, but I need the dual values for the constraints, and it's easier to access this separately. Otherwise I can't see how to do it without modifying a lot of stuff.