Converts zero values to nan values in selected columns

        :param \*cols: names of the colums
        :type \*cols: str, at least one

        :example: ``ds.zero_nan("mycol1", "mycol2")``

def zero_nan(self, *cols):
        """
        Converts zero values to nan values in selected columns

        :param \*cols: names of the colums
        :type \*cols: str, at least one

        :example: ``ds.zero_nan("mycol1", "mycol2")``
        """
        if len(cols) == 0:
            self.warning("Can not nan zero values if a column name "
                         "is not provided")
        df = self._zero_nan(*cols)
        if df is None:
            self.err("Can not fill zero values with nan")
            return
        self.df = df