feat: make kernels an enum that implements kernel

[morenol]

Fixes #

Checklist

• My branch is up-to-date with development branch.
• Everything works and tested on latest stable Rust.
• Coverage and Linting have been applied

Current behaviour

New expected behaviour

Change logs

[codecov-commenter]

Codecov Report

Merging #181 (5e9be36) into development (f8210d0) will increase coverage by 0.10%.
The diff coverage is 90.24%.

@@               Coverage Diff               @@
##           development     #181      +/-   ##
===============================================
+ Coverage        84.58%   84.69%   +0.10%     
===============================================
  Files               86       86              
  Lines             9974     9998      +24     
===============================================
+ Hits              8437     8468      +31     
+ Misses            1537     1530       -7     

Impacted Files	Coverage Δ
src/model_selection/kfold.rs	88.99% <ø> (ø)
src/model_selection/mod.rs	94.04% <ø> (ø)
src/svm/mod.rs	79.24% <77.77%> (-7.25%)	⬇️
src/model_selection/hyper_tuning/grid_search.rs	81.03% <100.00%> (+3.48%)	⬆️
src/svm/svc.rs	89.94% <100.00%> (+2.23%)	⬆️
src/svm/svr.rs	86.56% <100.00%> (+0.37%)	⬆️
src/metrics/mod.rs	32.43% <0.00%> (+8.10%)	⬆️

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[Mec-iS]

@morenol is this still a draft?

[Mec-iS]

This looks great though it would be better imo to have this to be merged directly in branch v0.5-wip as model_selection is already been ported. What do you thing @morenol ?
Could something similar to this also facilitate the porting of src/ensemble (see this comment)?

[Mec-iS]

Please resolve conflict with new code in development

[morenol]

I think that now that Kernel trait is object safe we can just use #228 approach instead