Add new data item.

        :param str id: Entry id within ``SDfile``.
        :param str key: Data item key.
        :param str value: Data item value.
        :return: None.
        :rtype: :py:obj:`None`.

def add_data(self, id, key, value):
        """Add new data item.

        :param str id: Entry id within ``SDfile``.
        :param str key: Data item key.
        :param str value: Data item value.
        :return: None.
        :rtype: :py:obj:`None`.
        """
        self[str(id)]['data'].setdefault(key, [])
        self[str(id)]['data'][key].append(value)