add state transition function tag to pomdpx model

        Return
        ---------------
        string containing the xml for state transition tag

def add_state_transition_function(self):
        """
        add state transition function tag to pomdpx model

        Return
        ---------------
        string containing the xml for state transition tag
        """
        state_transition_function = self.model['state_transition_function']
        for condition in state_transition_function:
            condprob = etree.SubElement(self.transition_function, 'CondProb')
            self.add_conditions(condition, condprob)
        return self.__str__(self.transition_function)[:-1]