reduce(callback, initialValue) {
        let accumulator = initialValue;
        this.forEach((cell, ri, ci) => {
            accumulator = callback(accumulator, cell, ri, ci, this);
        });

        return accumulator;
    }
Reduces the range to a single value accumulated from the result of a function called for each cell.
@param {Range~reduceCallback} callback - Function called for each cell in the range.
@param {*} [initialValue] - The initial value.
@returns {*} The accumulated value.