Add `solve_upper_triangular` to `CsrMatrix`
This allows a sparse matrix to be used for efficient solving with a dense LU decomposition.
Add CscBuilder
For partial construction of Csc matrices
Start working on actual LU factorization
Complete basic version of sparse LU factorization
Reformat to compile in old version
Add LU tests
Add upper triangular solve
Complete tests of Sparse LU factorization
julianknodt