add observation function tag to pomdpx model

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

def add_obs_function(self):
        """
        add observation function tag to pomdpx model

        Return
        ---------------
        string containing the xml for observation function tag
        """
        obs_function = self.model['obs_function']
        for condition in obs_function:
            condprob = etree.SubElement(self.observation_function, 'CondProb')
            self.add_conditions(condition, condprob)
        return self.__str__(self.observation_function)[:-1]