Refactor code to return interactions as sparse/dense Eigen matrices

[robomics]

Closes #171

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.92%. Comparing base (4075162) to head (4bad2f5).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #176      +/-   ##
==========================================
+ Coverage   84.85%   84.92%   +0.06%     
==========================================
  Files         117      119       +2     
  Lines       10037    10076      +39     
==========================================
+ Hits         8517     8557      +40     
+ Misses       1520     1519       -1     

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