Evaluator bug : service.py & client.py

The FLATland challenge round 2 requires remove_agents_at_target=True in the RailEnv. This is missing.

Decision to take: I guess many user will forget setting this new parameter. As we added this one, i see two options: (A) set it by default to True (B) remove the default value, then each user has to manually set the correct parameter.

Round 0, Round 1 it has to be set to zero, Round 2 to True.