docs: def save_elem_file(self, output):
          """Save elem.dat to file.
          The grid is saved as read in, i.e., with or without applied cutmck.
          If you want to change node coordinates, use self.nodes['raw']
  
          Parameters
          ----------
          filename: string
              output filename
          """
          with open(output, 'wb') as fid:
              self._write_elem_header(fid)
              self._write_nodes(fid)
              self._write_elements(fid)
              self._write_neighbors(fid)