docs: 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]