Add a __str__ function to the TrackGraph with num nodes and edges

[cmalinmayor]

Closes #81

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.94%. Comparing base (f6c9cba) to head (c619b58).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
motile/track_graph.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #123      +/-   ##
==========================================
- Coverage   93.06%   92.94%   -0.12%     
==========================================
  Files          30       30              
  Lines         721      723       +2     
==========================================
+ Hits          671      672       +1     
- Misses         50       51       +1     

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