union(dfToUnion) {
        if (!(dfToUnion instanceof DataFrame))
            throw new ArgumentTypeError(dfToUnion, "DataFrame");
        if (!arrayEqual(this[__columns__], dfToUnion[__columns__])) {
            throw new WrongSchemaError(
                dfToUnion[__columns__],
                this[__columns__]
            );
        }
        return this.__newInstance__(
            [...this, ...dfToUnion.restructure(this[__columns__])],
            this[__columns__]
        );
    }
Concat two DataFrames.
@param {DataFrame} dfToUnion The DataFrame to concat.
@returns {DataFrame} A new concatenated DataFrame resulting of the union.
@example
df.union(df2)