Reads the bedverb.mdv file.

def ImportBoarding(self, drop_off_file):
    "Reads the bedverb.mdv file."
    for trip_id, seq, code in \
    ReadCSV(drop_off_file, ['FRT_FID', 'LI_LFD_NR', 'BEDVERB_CODE']):
      key = (trip_id, int(seq) - 1)
      if code == 'A':
        self.pickup_type[key] = '1'  # '1' = no pick-up
      elif code == 'E':
        self.drop_off_type[key] = '1'  # '1' = no drop-off
      elif code == 'B' :
        # 'B' just means that rider needs to push a button to have the driver
        # stop. We don't encode this for now.
        pass
      else:
        raise ValueError('Unexpected code in bedverb.mdv; '
                         'FRT_FID=%s BEDVERB_CODE=%s' % (trip_id, code))