add shap value features on each estimator (depends on master branch of shap)

[heimengqi]

• add shap_values method on each of our estimators except for OrthoIV classes, which plan to add after we have a more complete version of subclasses
• Currently this PR depends on the master branch of shap package, not the released version
• Fix the comparison table

[kbattocchi]

Mostly looks good, but I added some feedback.

[vsyrgkanis]

Another general request: I think we should have a Explain Cate with SHAP.ipynb notebook that portrays all the functionality with SHAP. It seems important. But we could also do this post-release and prioritize all the other changes requested.

[vsyrgkanis]

All comments have been addressed!