centroid computation for nonconvex regions

[AhmetNSimsek]

addresses: #157

[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.61%. Comparing base (ccf29eb) to head (2698c90).
⚠️ Report is 46 commits behind head on main.

Files with missing lines	Patch %	Lines
siibra/volumes/volume.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #720      +/-   ##
==========================================
- Coverage   46.81%   46.61%   -0.20%     
==========================================
  Files          73       73              
  Lines        7729     7783      +54     
==========================================
+ Hits         3618     3628      +10     
- Misses       4111     4155      +44     

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