Fixes `cross_validate` method of `Regressor` class, introducing some breaking changes to inputs and outputs.
What's Changed
* Update notebooks by JohnGoertz in https://github.com/JohnGoertz/Gumbi/pull/1
* Merge v0.1.2 by JohnGoertz in https://github.com/JohnGoertz/Gumbi/pull/2
* Refactors and fixes cross_validate. See 4. by JohnGoertz in https://github.com/JohnGoertz/Gumbi/pull/5
**Full Changelog**: https://github.com/JohnGoertz/Gumbi/compare/v0.1.2...v0.1.3