Remove shap as optional dependency

[mmschlk]

Currently shap is used as a dependency for creating some of the plots in shapiq. Keeping shap as an optional dependency makes updating shapiq to newer Python versions and allowing shapiq to work with newer numpy versions quite stressful and tricky.

I think removing shap as imports anywhere in the project could be a good idea. For this we would need to re-create the plots in our own version. This could be more or less effort.

[hbaniecki]

Good idea.