sample(percentage) {
        const nRows = this.count() * percentage;
        let token = 0;
        return this.__newInstance__(
            iter(
                this.shuffle()[__rows__],
                (row) => {
                    token++;
                    return row;
                },
                () => token >= nRows
            ),
            this[__columns__]
        );
    }
Return a random sample of rows.
@param {Number} percentage A percentage of the orignal DataFrame giving the sample size.
@returns {DataFrame} A sample DataFrame
@example
df.sample(0.3)