Provide interface for more parallel solvers/preconditioners of ISTL

So far we have only the AMG-backend (parallel linear solver based on the ISTL AMG preconditioner and the ISTL BiCGSTAB solver). It would be great to add support for more preconditioners and solvers of ISTL.

The following combinations should be available:

• SSOR with BiCGSTAB
• ILU with BiCGSTAB
• ILU with GMRES

Are there any further wishes ?