Fixes problems in mdp.py (#918)
Fixes problems in mdp.py (#918)

* Added MDP2 class

* Updated loop termination condition in value_iteration

aimacode/aima-python,jo-tez/aima-python,Chipe1/aima-python,Chipe1/aima-python,aimacode/aima-python,jo-tez/aima-python