Add new ensemble methods crate: linfa-ensemble

[relf]

This PR supersedes PR #229 by addressing open review comments.
It adds a new crate for ensemble learning algorithms including a boostrap aggregation algorithm (aka bagging).

Co-authored-by: James Knight [email protected]
Co-authored-by: James Kay [email protected]

[codecov]

Codecov Report

Attention: Patch coverage is 26.66667% with 44 lines in your changes missing coverage. Please review.

Project coverage is 36.11%. Comparing base (335924e) to head (f5e1f6a).

Files with missing lines	Patch %	Lines
algorithms/linfa-ensemble/src/hyperparams.rs	14.81%	23 Missing ⚠️
algorithms/linfa-ensemble/src/algorithm.rs	36.36%	21 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #392      +/-   ##
==========================================
+ Coverage   36.02%   36.11%   +0.09%     
==========================================
  Files          99      101       +2     
  Lines        6496     6556      +60     
==========================================
+ Hits         2340     2368      +28     
- Misses       4156     4188      +32     

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