Use composition rather than inheritance

[tlambert03]

Rather than inheriting the cython graph, this PR moves to composition, where all python objects have a _cgraph object that they dispatch to

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.50649% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.03%. Comparing base (7842158) to head (a990aa6).
⚠️ Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatial_graph/_graph/graph_base.py	90.47%	6 Missing ⚠️
src/spatial_graph/_graph/views.py	95.00%	4 Missing ⚠️
src/spatial_graph/_rtree/rtree.py	96.29%	2 Missing ⚠️
src/spatial_graph/_spatial_graph.py	77.77%	2 Missing ⚠️
src/spatial_graph/_graph/graph.py	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #41      +/-   ##
==========================================
- Coverage   96.67%   95.03%   -1.64%     
==========================================
  Files          10       12       +2     
  Lines         331      383      +52     
==========================================
+ Hits          320      364      +44     
- Misses         11       19       +8     

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[codspeed-hq]

CodSpeed Performance Report

Merging #41 will degrade performances by 11.13%

_{Comparing tlambert03:composition (a990aa6) with main (7842158)}

Summary

❌ 1 regressions
✅ 5 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	test_roi_query_performance[100000]	1.7 ms	1.9 ms	-11.13%