sparse module: add support for umfpack, the sparse direct LU solver from suitesparse (as cholmod).
It seems to be even faster than SuperLU and it was much simpler to interface!
Well, the factorization is faster, but for the solve part, SuperLU is quite faster.

On the other hand the solve part represents only a fraction of the whole procedure.

Moreover, I bench random matrices that does not represents real cases, and I'm not sure at all I use both libraries with their best settings!