return table in RST format

def format_rst(self):
        """
        return table in RST format
        """
        res = ''
        num_cols = len(self.header)
        col_width = 25
        for _ in range(num_cols):
            res += ''.join(['=' for _ in range(col_width - 1)]) + ' ' 
        res += '\n'
        for c in self.header:
            res += c.ljust(col_width) 
        res += '\n'
        for _ in range(num_cols):
            res += ''.join(['=' for _ in range(col_width - 1)]) + ' ' 
        res += '\n'
        for row in self.arr:
            for c in row:
                res += self.force_to_string(c).ljust(col_width)
            res += '\n' 
        for _ in range(num_cols):
            res += ''.join(['=' for _ in range(col_width - 1)]) + ' ' 
        res += '\n'
        return res