Same as _check_stringify_year_row but for columns.

def _check_stringify_year_column(self, column_index):
        '''
        Same as _check_stringify_year_row but for columns.
        '''
        table_column = TableTranspose(self.table)[column_index]
        # State trackers
        prior_year = None
        for row_index in range(self.start[0]+1, self.end[0]):
            current_year = table_column[row_index]
            if not self._check_years(current_year, prior_year):
                return
            # Only copy when we see a non-empty entry
            if current_year:
                prior_year = current_year

        # If we have a title of years, convert them to strings
        self._stringify_column(column_index)