XGBoost serde support

[jamieyoung5]

Hey

The xgboost crate lacks serde attributes for Serialize and Deserialize. Is there a particular reason for this or is it something that is planned for future? I'd be happy to make these changes myself.

[Mec-iS]

Thanks for using smartcore.

please describe the use cases you have in mind for the feature you propose, then proceed and fill the issue template or PR template.

[cmmartin]

All of the other models implement Debug, Serialize and Deserialize except XGBoost. XGRegressorParameters should also implement the Default trait to match the other models

[Mec-iS]

yes, all the serialisation harness is optional under the serde flag and it is implemented on a need-to-have basis. Evidently no user needed serialisation for that model yet, indeed it would be nice to have it if you need it.

[cmmartin]

Yes of course. Was just clarifying the request. @jamieyoung5 offered to add it