add initial belief tag to pomdpx model

        Return
        ---------------
        string containing the xml for initial belief tag

def add_initial_belief(self):
        """
        add initial belief tag to pomdpx model

        Return
        ---------------
        string containing the xml for initial belief tag
        """
        initial_belief = self.model['initial_state_belief']
        for condition in initial_belief:
            condprob = etree.SubElement(self.initial_belief, 'CondProb')
            self.add_conditions(condition, condprob)
        return self.__str__(self.initial_belief)[:-1]