Overload save and restore

[ablaom]

Resolves #207

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.42%. Comparing base (da15fb3) to head (70445e7).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #208      +/-   ##
==========================================
+ Coverage   87.38%   87.42%   +0.03%     
==========================================
  Files          13       13              
  Lines         658      660       +2     
==========================================
+ Hits          575      577       +2     
  Misses         83       83              

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[ablaom]

@paulotrefosco Be great if you can confirm this MLJTuning.jl branch resolves your issue with wrapping XGBoost models in TunedModel.

You can add this branch to your environment with using Pkg; Pkg.add(name="MLJTuning", rev="save-restore")

[OkonSamuel]

Looks good to me, except for the minor change