add reward function tag to pomdpx model

        Return
        ---------------
        string containing the xml for reward function tag

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]