feat: add Density class

[carlos-adir]

This PR adds a new class Density

It's the return of functions named density that all SubSetR2 classes implements.

It was added as pre-requisite of adding Lazy evaluators, named #63

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.54%. Comparing base (c21a57c) to head (4f98e71).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #74      +/-   ##
==========================================
- Coverage   98.56%   98.54%   -0.02%     
==========================================
  Files          27       28       +1     
  Lines        2514     2620     +106     
==========================================
+ Hits         2478     2582     +104     
- Misses         36       38       +2     

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