Add function to turn TrackGraph to networkx graph

[cmalinmayor]

Proposed Change

Add function to turn TrackGraph into networkx graph and preserve attributes. This should probably go into the TrackGraph itself in motile, but at least it is here for now.

Checklist

Go through these things before merge. Actions should run automatically to test them, but for information on how to run locally, see CONTRIBUTING.md.

• [x] I have added tests that prove that my feature works in various situations or tests the bugfix (if applicable).
• [x] I have checked that the tests pass and I maintained or improved test coverage (if applicable).
• [x] I have written docstrings and checked that they render correctly in the documentation build.
• [x] I have checked that mypy type checking passes.

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