Implement simple kernel with equal weight for all neighbors

[Marius1311]

Could be useful for spatial smoothing. Very basic.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.22%. Comparing base (b95cae7) to head (253c46d).
⚠️ Report is 180 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
+ Coverage   85.17%   85.22%   +0.04%     
==========================================
  Files           7        7              
  Lines         607      609       +2     
==========================================
+ Hits          517      519       +2     
  Misses         90       90              

Files with missing lines	Coverage Δ
src/cellmapper/cellmapper.py	87.71% <100.00%> (ø)
src/cellmapper/knn.py	79.65% <100.00%> (+0.23%)	⬆️

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