[Feature] `WithoutLiersCV` model selection

[FBruzzesi]

Description

Introduces WithoutLiersCV as discussed in #307. To be able to follow different cross validation strategies, the idea is to take a CV object as input and exclude the anomalous samples from the training indexes. All the splitting logic is delegated to the cv object.

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality): Fixes #307
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [X] My code follows the style guidelines (flake8)
• [X] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation (also to the readme.md)
• [X] I have added tests that prove my fix is effective or that my feature works
• [X] I have added tests to check whether the new feature adheres to the sklearn convention
• [X] New and existing unit tests pass locally with my changes