Convert columns to StrLs if either very large or in the
        convert_strl variable

def _convert_strls(self, data):
        """Convert columns to StrLs if either very large or in the
        convert_strl variable"""
        convert_cols = [
            col for i, col in enumerate(data)
            if self.typlist[i] == 32768 or col in self._convert_strl]

        if convert_cols:
            ssw = StataStrLWriter(data, convert_cols)
            tab, new_data = ssw.generate_table()
            data = new_data
            self._strl_blob = ssw.generate_blob(tab)
        return data