reduce(func, init) {
        return typeof init === "undefined"
            ? this[__rows__].reduce((p, n) => func(p, n))
            : this[__rows__].reduce((p, n) => func(p, n), init);
    }
Reduce DataFrame into a value.
@param {Function} func The reduce function taking 2 parameters, previous and next.
@param [init] The initial value of the reducer.
@returns A reduced value.
@example
df.reduce((p, n) => n.get('column1') + p, 0)
df2.reduce((p, n) => (
         n.set('column1', p.get('column1') + n.get('column1'))
          .set('column2', p.get('column2') + n.get('column2'))
))