Determine the path to the correct calibration cache file to use.

def calibration_cache_path(self):
    """
    Determine the path to the correct calibration cache file to use.
    """
    if self.__ifo and self.__start > 0:
        cal_path = self.job().get_config('calibration','path')

        # check if this is S2: split calibration epochs
        if ( self.__LHO2k.match(self.__ifo) and
          (self.__start >= 729273613) and (self.__start <= 734367613) ):
          if self.__start < int(
            self.job().get_config('calibration','H2-cal-epoch-boundary')):
            cal_file = self.job().get_config('calibration','H2-1')
          else:
            cal_file = self.job().get_config('calibration','H2-2')
        else:
            # if not: just add calibration cache
            cal_file = self.job().get_config('calibration',self.__ifo)

        cal = os.path.join(cal_path,cal_file)
        self.__calibration_cache = cal
    else:
       msg = "IFO and start-time must be set first"
       raise CondorDAGNodeError, msg