code: def open_mdf(self):
          
          mdf_path = self.mdf_path
          if not mdf_path:
              logger.critical("%s didn't produce a MDF file in %s" % (self, self.outdir))
              return None
  
          # Open the DFF file and add its data to results.out
          from abipy.electrons.bse import MdfFile
          try:
              return MdfFile(mdf_path)
          except Exception as exc:
              logger.critical("Exception while reading MDF file at %s:\n%s" % (mdf_path, str(exc)))
              return None
docs: Open the MDF file located in the in self.outdir.
          Returns :class:`MdfFile` object, None if file could not be found or file is not readable.