Add local explainability implementation

[willfleury]

Add local explainability example for Python XGBoost using the following package

https://pypi.org/project/alibi/

Unlike shap, this allows to produce the explainer once (training) and then save it (serialize) so you can load it later and use it for individual prediction explaination.

Add the explain endpoint in the scoring.py->ModelScorer.explain()

[CS250073]

KSI've started on this, but it's getting way above my DS skills, maybe @KS2907 you can take over and finish it, thanks!

Branch is #67

Some info:
https://github.com/SeldonIO/alibi/blob/master/examples/path_dependent_tree_shap_adult_xgb.ipynb

[KS2907]

@Christian-TD I never got a notification for this comment somehow. Saw this while looking through the issues under demo models. @willfleury is it still an active lead that you want me to explore?

[willfleury]

@KS2907 sure - have a look at it and lets chat.