return a dictionary of {col -> width}

def _get_column_widths(self):
        """return a dictionary of {col -> width}"""
        col_widths = {}
        for table, (row, col) in self.__tables.values():
            for colname, width in table.column_widths.items():
                ic = col + table.get_column_offset(colname)
                current_width = col_widths.setdefault(ic, width)
                col_widths[ic] = max(width, current_width)
        return col_widths