docs: 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()