[preconditioners] Add three preconditioners
- Thread-parallel Jacobi smoother/preconditioner
- Thread-parallel SOR/SSOR with coloring
- They can be selected via the factory
- They can be used with AMG as smoother (but not unfortunately not via the factory yet) The reason being that Dune currently hard-coded the selection of available smoothers. We would have to write our own AMGCreator to include them. Probably better to find an upstream solution.
The tests added for the richards test only add a few seconds in runtime because the solver factory is used anyways. Because the grid is quite small in the test multi-threading actually makes them slightly slower. But for larger grids there is a speedup.
Extracted from !3240
Edited by Timo Koch