sortColumns(compareFunction) {
                if (compareFunction === undefined) compareFunction = compareNumbers;
                for (var i = 0; i < this.columns; i++) {
                    this.setColumn(i, this.getColumn(i).sort(compareFunction));
                }
                return this;
            }
Sorts the columns (in place)
@param {function} compareFunction - usual Array.prototype.sort comparison function
@return {Matrix} this