bisect(percentage) {
        const nRows = this.count() * percentage;
        let token = 0;
        const restRows = [];
        return [
            this.__newInstance__(
                iter(this.shuffle()[__rows__], (row) => {
                    if (token < nRows) {
                        token++;
                        return row;
                    }
                    restRows.push(row);
                }),
                this[__columns__]
            ),
            this.__newInstance__(restRows, this[__columns__])
        ];
    }
Randomly split a DataFrame into 2 DataFrames.
@param {Number} percentage A percentage of the orignal DataFrame giving the first DataFrame size. The second takes the rest.
@returns {Array} An Array containing the two DataFrames. First, the X% DataFrame then the rest DataFrame.
@example
const [30DF, 70DF] = df.bisect(0.3)