*Improve QR factorization by replacing CXSPARSE code with SPQR code, and make the linear solve return 2-norm solutions for ill-conditioned matrices based on this new code
Implement fourth argument to the sprand and sprandn, and addition arguments to sprandsym that the leading brand implements.
*Sparse logical indexing in idx_vector class so that something like
'a=sprandn(1e6,1e6,1e-6); a(a<1) = 0 ' won't cause a memory overflow.
*Other missing Functions