Save :class:`~mwtab.mwtab.MWTabFile` in `mwtab` formatted string.

        :return: NMR-STAR string.
        :rtype: :py:class:`str`

def _to_mwtab(self):
        """Save :class:`~mwtab.mwtab.MWTabFile` in `mwtab` formatted string.

        :return: NMR-STAR string.
        :rtype: :py:class:`str`
        """
        mwtab_str = io.StringIO()
        self.print_file(mwtab_str)
        return mwtab_str.getvalue()