code: def set_observatory(self,obs):
      
      self.add_var_opt('observatory',obs)
      self.__observatory = str(obs)
      self.__set_output()
docs: Set the IFO to retrieve data for. Since the data from both Hanford
      interferometers is stored in the same frame file, this takes the first
      letter of the IFO (e.g. L or H) and passes it to the --observatory option
      of LSCdataFind.
      @param obs: IFO to obtain data for.