Same as _stringify_row but for columns.

def _stringify_column(self, column_index):
        '''
        Same as _stringify_row but for columns.
        '''
        table_column = TableTranspose(self.table)[column_index]
        prior_cell = None
        for row_index in range(self.start[0], self.end[0]):
            cell, changed = self._check_interpret_cell(table_column[row_index], prior_cell, row_index, column_index)
            if changed:
                table_column[row_index] = cell
            prior_cell = cell