add serde support for XGRegressor

[cmmartin]

Fixes #325

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

XGRegressor does not implement the Serialize/Deserialize or Debug traits

New expected behaviour

XGRegressor implements the Serialize/Deserialize and Debug traits

Change logs

[Mec-iS]

Please see the failures in the CI checks. There are missing imports.
Have you tried to build your code with cargo build or run the tests with cargo test?

Please follow the guidelines in CONTRIBUTING.md

• Formatting: run rustfmt src/*.rs to apply automatic formatting
• Linting: clippy is used with command cargo clippy --all-features -- -Drust-2018-idioms -Dwarnings
• Coverage (optional): tarpaulin is used with command cargo tarpaulin --out Lcov --all-features -- --test-threads 1
• Testing: multiple test pipelines are run for different targets

[cmmartin]

My fault. I did run build and test and they worked fine but I forgot this is behind a feature flag. I updated the code and cargo test --all-features and cargo build --all-features are passing now

[Mec-iS]

thanks. will go on the next release 👍

[Mec-iS]

thanks. will go on the next release 👍