apply(callback) {
                if (typeof callback !== 'function') {
                    throw new TypeError('callback must be a function');
                }
                var ii = this.rows;
                var jj = this.columns;
                for (var i = 0; i < ii; i++) {
                    for (var j = 0; j < jj; j++) {
                        callback.call(this, i, j);
                    }
                }
                return this;
            }
Applies a callback for each element of the matrix. The function is called in the matrix (this) context.
@param {function} callback - Function that will be called with two parameters : i (row) and j (column)
@return {Matrix} this