sumrows does not require matrix uncompression
linearsystem has a optional sparsity pattern instead of initialization value (which was not respected anyway for sparse matrices)
related changes to previous commit
some fixes to matrix initialization/clearing, bugfix in assigning from vector of size 1, improved compression and ensure_structural_symmetry, and added a direct (row-oriented only) compression initialization from sparsity pattern (based on old code, but works very well)