Explicit support for Regression, performed major refactoring of tests, removed unused code and updated notebooks to work (again).

[ReinierKoops]

This PR depends on the PR to be accepted: https://github.com/ing-bank/probatus/pull/242

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The PR fixes the following:

• Removed unused code and so partially fixed: https://github.com/ing-bank/probatus/issues/245
• Updates all notebooks so they are all working: https://github.com/ing-bank/probatus/issues/246
• Add explicit support for regressors and multiclass: : https://github.com/ing-bank/probatus/issues/241 & : https://github.com/ing-bank/probatus/issues/169
• Updated the yaml files for github actions. Now we have a weekly cronjob (instead of daily) to test the notebooks.
• Enable tests which were previously failing or only enabled in certain circumstances (much has changed the last 3 years with regards of adoption of the Mac ARM architecture support)
• Removed most (if not all except one on purpose) of the copyright notice in code.

[ReinierKoops]

@PaulZhutovsky @adri0 the branch should be cleaned up a bit now (I rebased it), so reviewing should be (hopefully) easier.