Integrating Stokes solvers from branch burk/ELWMS.

Nils Kohlrequested to merge
kohl/stokes-solvers into master

Mostly taking files that have been introduced by Andi Burkhart on the burk/ELWMS branch (specifically at 0a3cdd49).

This MR introduces new and more modular Stokes preconditioners and adds necessary features to use them with the new generated operators.

Merge request reports