Adds a single value (cell) to a worksheet at (row, col).
        Return the (row, col) where the value has been put.

        :param value: Value to write to the sheet.
        :param row: Row where the value should be written.
        :param col: Column where the value should be written.

def add_value(self, value, row, col):
        """
        Adds a single value (cell) to a worksheet at (row, col).
        Return the (row, col) where the value has been put.

        :param value: Value to write to the sheet.
        :param row: Row where the value should be written.
        :param col: Column where the value should be written.
        """
        self.__values[(row, col)] = value