[SPARK-2851] [mllib] DecisionTree Python consistency update

Added 6 static train methods to match Python API, but without default arguments (but with Python default args noted in docs).

Added factory classes for Algo and Impurity, but made private[mllib].

CC: @mengxr @dorx  Please let me know if there are other changes which would help with API consistency---thanks!
minor: `stringToAlgo` -> `fromString`? In the code, it might be more natural to read

~~~
Algo.fromString("classification")
~~~

~~~
Algo.stringToAlgo("classification")
~~~