docs: def add_reward_function(self):
          """
          add reward function tag to pomdpx model
  
          Return
          ---------------
          string containing the xml for reward function tag
          """
          reward_function = self.model['reward_function']
          for condition in reward_function:
              condprob = etree.SubElement(self.reward_function, 'Func')
              self.add_conditions(condition, condprob)
          return self.__str__(self.reward_function)[:-1]