slice(startIndex, endIndex) {
        return this.__newInstance__(
            this[__rows__].slice(
                startIndex || undefined,
                endIndex || undefined
            ),
            this[__columns__]
        );
    }
Create a new subset DataFrame based on given indexs. Similar to Array.slice.
@param {Number} [startIndex=0] The index to start the slice (included).
@param {Number} [endIndex=this.count()] The index to end the slice (excluded).
@returns {DataFrame} The subset DataFrame.
@example
df2.slice()
df2.slice(0)
df2.slice(0, 20)
df2.slice(10, 30)