docs: def _get_field(xdmf_file, data_item):
      """Extract field from data item."""
      shp = _get_dim(data_item)
      h5file, group = data_item.text.strip().split(':/', 1)
      icore = int(group.split('_')[-2]) - 1
      fld = _read_group_h5(xdmf_file.parent / h5file, group).reshape(shp)
      return icore, fld