outerJoin(dfToJoin, columnNames) {
        return this.fullJoin(dfToJoin, columnNames);
    }
Join two DataFrames with outer mode.
@param {DataFrame} dfToJoin The DataFrame to join.
@param {String | Array} columnNames The selected columns for the join.
@returns {DataFrame} The joined DataFrame.
@example
df2.outerJoin(df2, 'id')
df2.join(df2, 'id', 'outer')