Move to enum Kernels instead of Structs for hypertuning

[montanalow]

SVCSearchParameters currently uses a Vec for kernels, which means you can't search across multiple kernel types at the same time. This PR converts K to an enum Kernel type, instead of a generic type, so that multiple kinds of kernels may be compared in the same grid search.

[montanalow]

This is a breaking change, so may require more thought on the approach.

[codecov-commenter]

Codecov Report

Merging #169 (fca9f35) into development (403d3f2) will increase coverage by 0.06%.
The diff coverage is 85.89%.

@@               Coverage Diff               @@
##           development     #169      +/-   ##
===============================================
+ Coverage        84.40%   84.47%   +0.06%     
===============================================
  Files               86       86              
  Lines             9921     9927       +6     
===============================================
+ Hits              8374     8386      +12     
+ Misses            1547     1541       -6     

Impacted Files	Coverage Δ
src/model_selection/hyper_tuning.rs	0.00% <ø> (ø)
src/svm/mod.rs	89.47% <84.00%> (+2.98%)	⬆️
src/svm/svr.rs	86.46% <84.61%> (+0.27%)	⬆️
src/svm/svc.rs	88.91% <87.50%> (+1.19%)	⬆️
src/optimization/first_order/lbfgs.rs	92.85% <0.00%> (-1.59%)	⬇️
src/math/num.rs	82.69% <0.00%> (+3.84%)	⬆️

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[Mec-iS]

please add the breaking change to the CHANGESLOG

[morenol]

I think that having the Kernel trait give us more flexibility. More people could implement their own Kernel and use it without troubles. I think that what we could do for ease of use is to convert Kernels into an enum and implement Kernel for Kernels.

I tried that approach in #181

Let me know if that is enough for your use case @montanalow

On the other hand, it looks like the original test is failing. Can you check that in your branch?
I got:

thread 'model_selection::hyper_tuning::tests::svm_check' panicked at 'index out of bounds: the len is 0 but the index is 0', src/svm/svc.rs:724:28

[Mec-iS]

Please adapt these changes to the new code in development.

Kernel is now an object-safe trait. Making an enum of Kernels had been a problem for me because a match expression needs to return the same type. I tried different options but for sure there is something I am missing:

pun enum Kernels {
    Linear(LinearKernel),
    RBF(RBFKernel)
    ...
}

match kernel {
    Linear => LinearKernel::default(),  // this returns a LinearKernel type
    RBF => RBFkernel::default(),       // this returns a RBFKernel type
    ...
}