docs: def fei_metadata(self):
          """Return FEI metadata from SFEG or HELIOS tags as dict."""
          if not self.is_fei:
              return None
          tags = self.pages[0].tags
          if 'FEI_SFEG' in tags:
              return tags['FEI_SFEG'].value
          if 'FEI_HELIOS' in tags:
              return tags['FEI_HELIOS'].value
          return None