Multi class shap rfecv

[lproth]

Re https://github.com/ing-bank/probatus/issues/169 I have added support for multi-class classification and ensured that regression will still work. I haven't written any unit tests because I'm not familiar with how pytest works, but all that should be needed is to re-run the existing tests with a multi-class target instead of binary.

[Matgrb]

Also please have a look what has changed in, just to make sure your PR is aligned #175

[lproth]

Hi @Matgrb, can we merge this?

[ReinierKoops]

Hi @lproth , sorry for the delayed response and thank you for your contribution, could you update this branch to be compatible with the latest changes on the main?

[ReinierKoops]

This can only be merged when some unit tests are added.