subMatrixView(startRow, endRow, startColumn, endColumn) {
                return new MLMatrixSubView(this, startRow, endRow, startColumn, endColumn);
            }
Returns a view of a submatrix giving the index boundaries
@param {number} startRow - first row index of the submatrix
@param {number} endRow - last row index of the submatrix
@param {number} startColumn - first column index of the submatrix
@param {number} endColumn - last column index of the submatrix
@return {MatrixSubView}