Export TrackGraph to networkx

[cmalinmayor]

Closes #85
In testing this, I discovered some inconsistency/unspecificity in our API for specifying attributes for hyperedges.

This implementation enforces that the attributes should be on the hypernode on ingest, and will also be written to the hypernode on output (or nowhere if the edges are flattened). This is a change from what was previously implemented on ingest (attributes specified on any of the networkx edges of the hypernode would be smushed onto the new hyperedge in the track graph in unspecified order, and attributes on the hypernode were ignored, despite that being how attributes were specified in our hyperedge testing fixture.)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.61%. Comparing base (2aca13f) to head (f4d7a68).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #143      +/-   ##
==========================================
+ Coverage   93.00%   93.61%   +0.61%     
==========================================
  Files          30       30              
  Lines         729      752      +23     
==========================================
+ Hits          678      704      +26     
+ Misses         51       48       -3     

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