Add TreeShap explainer

[ukclivecox]

What this PR does / why we need it:
Adds TreeShap explainer

• Updates explainer wrapper
• Adds yaml example to explainer_examples notebook

Which issue(s) this PR fixes:

Fixes #2361

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[review-notebook-app]

Check out this pull request on 

Review Jupyter notebook visual diffs & provide feedback on notebooks.

---

Powered by ReviewNB

[seldondev]

Tue Sep 1 18:20:27 UTC 2020
The logs for [pr-build] [1] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2363/1.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2363 --build=1

[seldondev]

Tue Sep 1 18:20:34 UTC 2020
The logs for [lint] [2] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2363/2.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2363 --build=2

[axsaucedo]

Nice - seems like Alibi Explain is now pretty much feature-covered in SC
/approve

[seldondev]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: axsaucedo

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [axsaucedo]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[seldondev]

Wed Sep 2 07:11:52 UTC 2020
The logs for [pr-build] [3] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2363/3.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2363 --build=3

[seldondev]

Wed Sep 2 07:12:05 UTC 2020
The logs for [lint] [4] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2363/4.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2363 --build=4