Set an index to the main dataframe

        :param col: column name where to index from
        :type col: str

        :example: ``ds.index("mycol")``

def index(self, col: str):
        """
        Set an index to the main dataframe

        :param col: column name where to index from
        :type col: str

        :example: ``ds.index("mycol")``
        """
        df = self._index(col)
        if df is None:
            self.err("Can not create index")
            return
        self.df = df