Add explain and stats option to Python DSL

This would allow users to see the plans and statistics for a given evaluation of Python DSL. Since this is a starter task, I would give some hints:
1. Extend "def eval(outputs, outputDF=False, execute=True):" method to accept explain and statistics options and pass them before "results = matrix.ml.execute(matrix.script)".
Here are the relevant links:
https://github.com/apache/incubator-systemml/blob/master/src/main/python/systemml/defmatrix.py#L290
https://github.com/apache/incubator-systemml/blob/master/src/main/python/systemml/mlcontext.py#L223

2. Expose these options through user facing APIs: eval() and methods that invoke eval (such as toPandas, toNumPyArray, toDataFrame, etc)

3. Update the documentation: https://apache.github.io/incubator-systemml/beginners-guide-python