Add Sklearn API

[joshdunnlime]

[x] add sklearn regressor based on XGBRegressor
[x] add some general tests based on tests_model
[x] move the optional extra SHap package imports to where the function is called*

[x] improve test coverage
[x] add documentation
[x] examples regressor

[ ] improve documentation

• the sklearn module imports from the model module. This seems to throw an ImportError for SHap when not installed.

[joshdunnlime]

@StatMixedML The majority of functionality available in XGBoost and XGBoostLSS is now available in the XGBoostLSS Sklearn API. I think now would be a great time to get a bit of a review, even if just high-level thought and comments.

[StatMixedML]

@joshdunnlime Can you please move the PR to branch sklearn_integration, thanks.