\returns an expression of the matrix Q as products of sparse Householder reflectors. * The common usage of this function is to apply it to a dense matrix or vector * \code * VectorXd B1, B2; * // Initialize B1 * B2 = matrixQ() * B1; * \endcode * * To get a plain SparseMatrix representation of Q: * \code * SparseMatrix Q; * Q = SparseQR<SparseMatr