Gauß-Seidel preconditioner

[upsj]

We already have our own triangular solver kernels, extending them to a Gauß-Seidel kernel should be straightforward.

[greole]

https://github.com/JakubTrzcskni/ginkgo/tree/gauss-seidel-precond is working on a Gauß-Seidel preconditioner

[yhmtsai]

It will also be handled in #1633