Rename and tighten type definitions

[funkey]

We had conflicting definitions of what a hyperedge is in different parts of code and documentation.

This PR defines a hyperedge as a 2-tuple of tuples (in-nodes, out-nodes). Documentation updated to reflect this.

There is now a TrackGraph.is_hyperedge that can be used to test whether an edge is a hyperedge (should be preferred over manual isinstance calls). This function will also yell if a tuple is neither a normal edge nor a hyperedge as defined above.

Renamed type definitions while at it as well to be shorter and more descriptive.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 91.70%. Comparing base (825a461) to head (fb955bc).

Files	Patch %	Lines
motile/_types.py	0.00%	5 Missing ⚠️
motile/track_graph.py	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #112      +/-   ##
==========================================
- Coverage   92.15%   91.70%   -0.46%     
==========================================
  Files          30       30              
  Lines         701      711      +10     
==========================================
+ Hits          646      652       +6     
- Misses         55       59       +4     

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[cmalinmayor]

@tlambert03 In your opinion, should we consider this codecov complaint seriously? (I fixed the docs issue and otherwise we are ready to merge, I think)

[tlambert03]

No, based on the diff, it's just telling you that this code isn't covered (and probably wasn't already before this PR). It's a slight "problem" per se, but not one specific to this PR