subMatrix(startRow, endRow, startColumn, endColumn) {
                util.checkRange(this, startRow, endRow, startColumn, endColumn);
                var newMatrix = new this.constructor[Symbol.species](endRow - startRow + 1, endColumn - startColumn + 1);
                for (var i = startRow; i <= endRow; i++) {
                    for (var j = startColumn; j <= endColumn; j++) {
                        newMatrix[i - startRow][j - startColumn] = this.get(i, j);
                    }
                }
                return newMatrix;
            }
Returns a subset of the matrix
@param {number} startRow - First row index
@param {number} endRow - Last row index
@param {number} startColumn - First column index
@param {number} endColumn - Last column index
@return {Matrix}