docs: 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]