Implement Mod for Network Design Problem

[stevedwards]

Description

Implements #36

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:
  • Problem specification with description tab and optimzation model tab
  • Example of the input data format (use gurobi_optimods.datasets for loading data)
  • Runnable code example
  • Presentation of solutions

[gglockner]

I wanted to comment on the idea of networkx vs Pandas:

• networkx is the preferred API for graph algorithms. Anyone working on pure network applications is likely to use networkx.
• pandas is the preferred tool for data scientists and is far more popular than networkx.

So which API should be chosen should depend on who is the target for the tool.

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