Adding Scikit-Learn ExtraTreesRegressor to allowed models for TreeExplainer

[Deathn0t]

Hello,

I am opening this PR to add the ExtraTreesRegressor as part of allowed models for the TreeExplainer.

I noticed the ExtraTreesClassifier already added is not part of the tests because it is not part of the TREE_MODEL_FIXTURES list. Therefore, I did not add it. Let me know if you prefer to add it.

I tested the TreeExplainer live with ExtraTreesRegressor and it works.

[Deathn0t]

I pushed the fix for the test that failed.

I just found that issue #277 corresponds to this PR.

According to my knowledge, RandomForestPOSTFIX and ExtraTreesPOSTFIX in Scikit-Learn are based on the same code DecisionTreePOSTFIX. The difference is mostly in fixing splitter="best" for RandomForestPOSTFIX and splitter="random" for ExtraTreesPOSTFIX. Here is a hint to that:

• ExtraTreesClassifier uses ExtraTreeClassifier: (code in scikit-learn)
• ExtraTreeClassifier is a subclass of DecisionTreeClassifier with the splitter="random": (code in scikit-learn)