code: def open_sigres(self):
          
          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
docs: 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.