code: def is_metal(self, efermi_tol=1e-4):
          
          for spin, values in self.bands.items():
              for i in range(self.nb_bands):
                  if np.any(values[i, :] - self.efermi < -efermi_tol) and \
                          np.any(values[i, :] - self.efermi > efermi_tol):
                      return True
          return False
docs: Check if the band structure indicates a metal by looking if the fermi
          level crosses a band.
  
          Returns:
              True if a metal, False if not