Open the SIGRES file located in the in self.outdir.
        Returns :class:`SigresFile` object, None if file could not be found or file is not readable.

def open_sigres(self):
        """
        Open the SIGRES file located in the in self.outdir.
        Returns :class:`SigresFile` object, None if file could not be found or file is not readable.
        """
        sigres_path = self.sigres_path

        if not sigres_path:
            logger.critical("%s didn't produce a SIGRES file in %s" % (self, self.outdir))
            return None

        # Open the SIGRES file and add its data to results.out
        from abipy.electrons.gw import SigresFile
        try:
            return SigresFile(sigres_path)
        except Exception as exc:
            logger.critical("Exception while reading SIGRES file at %s:\n%s" % (sigres_path, str(exc)))
            return None