[Proof of Concept] Use symmetry to speed up the main loop

[efaulhaber]

Here, I skip half the neighbors and use symmetry to include them.
Note that this only works when the two systems are identical, so for example only for fluid-fluid interaction and not for fluid-neighbor interaction.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.22%. Comparing base (fde9a79) to head (c213dbf).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
+ Coverage   88.15%   88.22%   +0.07%     
==========================================
  Files          15       15              
  Lines         498      501       +3     
==========================================
+ Hits          439      442       +3     
  Misses         59       59              

Flag	Coverage Δ
unit	88.22% <100.00%> (+0.07%)	⬆️

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