Converting from NumPy array to list in mimic_explainer.py:_save()

[GraceBrigham]

Problem: When using _save() in mimic_explainer.py to serialize my explainable model I ran into the TypeError "Object of type ndarray is not JSON serializable." My issue was that the classes parameter I used in the constructor was a ndarray and not a list.

Solution: In the future, it might be nice if ndarrays are automatically converted to lists to avoid the serialization error in the future.

Thanks!