Helper for :meth:`consume_next`.

def _save_current_cell(self):
        """Helper for :meth:`consume_next`."""
        row, cell = self._row, self._cell
        family = row._cells.setdefault(cell.family_name, {})
        qualified = family.setdefault(cell.qualifier, [])
        complete = Cell.from_pb(cell)
        qualified.append(complete)
        self._cell, self._previous_cell = None, cell