code: def sigma_from_site_prop(self):
          
          num_coi = 0
          if None in self.site_properties['grain_label']:
              raise RuntimeError('Site were merged, this property do not work')
          for tag in self.site_properties['grain_label']:
              if 'incident' in tag:
                  num_coi += 1
          return int(round(self.num_sites / num_coi))
docs: This method returns the sigma value of the gb from site properties.
          If the GB structure merge some atoms due to the atoms too closer with
          each other, this property will not work.