Improving single node API

[JoOkuma]

Hey @TeunHuijben,

I created this PR based on our discussion last week.

I have mixed feelings about using __getitem__ of BaseGraph, I like the interface, but it's hard to generalize to edges.
Any suggestions? Maybe node(node_id=<VALUE>)?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.02%. Comparing base (a1cd2c3) to head (cec39a2).

Files with missing lines	Patch %	Lines
src/tracksdata/graph/_base_graph.py	77.77%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #129      +/-   ##
==========================================
- Coverage   88.08%   88.02%   -0.06%     
==========================================
  Files          49       49              
  Lines        3214     3232      +18     
  Branches      541      542       +1     
==========================================
+ Hits         2831     2845      +14     
- Misses        228      231       +3     
- Partials      155      156       +1     

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