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.

def open_mdf(self):
        """
        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.
        """
        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