Treeinterpreter class interface

[geoochi]

After Optimized mean calculation routine in _predict_tree and _predict_forest:

1. Used Treeinterpreter class as a user interface, in which:

   • model, X and joint_contribution needs to be passed in during initialization.
   • user can only use predict function
   • only tree can be passed through class methods.

2. Used ThreadPoolExecutor to optimize mean calculation routine. When self.joint_contribution == False:

   • mean prediction, biases and contributions set to class attributes
   • _predict_tree function caculate results and adds to these three class attributes without return