message: start with state evaluator
diff: -
  +from bark.world.evaluation import EvaluatorGoalReached
  class StateEvaluator:
  def get_evaluation(self, world):
  return # reward, done, info
  + def add_world_evaluators(self, world)
  + return # world
  +
  class GoalReached(StateEvaluator):
  def get_evaluation(self, world):
  + def add_world_evaluators(self, world):
  + evaluator = EvaluatorGoalReached(agent.id)
  + world.add_evaluator("success", evaluator)
  +
  