Enhance partition (#16)

#15 Create impurity function in Stree (consistent name, same criteria as other splitter parameter)
Create test for the new function
Update init test
Update test splitter parameters
Rename old impurity function to partition_impurity
close #15
* Complete implementation of splitter_type = impurity with tests
Remove max_distance & min_distance splitter types

* Fix mistake in computing multiclass node belief
Set default criterion for split to entropy instead of gini
Set default max_iter to 1e5 instead of 1e3
change up-down criterion to match SVC multiclass
Fix impurity method of splitting nodes
Update jupyter Notebooks

setup.py

@@ -1,6 +1,6 @@
 import setuptools
 
-__version__ = "0.9rc5"
+__version__ = "0.9rc6"
 __author__ = "Ricardo Montañana Gómez"
 
 
@@ -25,7 +25,7 @@ setuptools.setup(
     classifiers=[
         "Development Status :: 4 - Beta",
         "License :: OSI Approved :: MIT License",
-        "Programming Language :: Python :: 3.7",
+        "Programming Language :: Python :: 3.8",
         "Natural Language :: English",
         "Topic :: Scientific/Engineering :: Artificial Intelligence",
         "Intended Audience :: Science/Research",