add standalone visualizer v1

[TDebonoWise]

Adding a "standalone" (i.e., currently not directly interacting with auto-causality) visualizer class. Mode of interaction with auto-causality tbd.

Currently implements the following plots that are floating around in various notebooks as methods:

• SHAP summary plot
• ATE/norm_erupt per estimator per split
• Group ATE plot (single & multi-treatment); single-treatment plot from notebooks extended to account for multi-treatment

Also mimics some methods from auto_causality.scorer (ate, naive_ate) as a result of being "standalone".

Open to-dos:

• [ ] decide on mode of interaction, i.e., how to integrate with auto-causality
• [ ] make plots themselves a bit nicer and allow for more flexibility/customization
• [ ] add more plots
• [ ] write tests (?)