N-step returns for rainbow (#67)

* n_step returns for rainbow
* Rename CartPole_PPO -> CartPole_ClippedPPO

rl_coach/base_parameters.py

@@ -166,6 +166,9 @@ class AlgorithmParameters(Parameters):
         # intrinsic reward
         self.scale_external_reward_by_intrinsic_reward_value = False
 
+        # n-step returns
+        self.n_step = -1  # calculate the total return (no bootstrap, by default)
+      
         # Distributed Coach params
         self.distributed_coach_synchronization_type = None