Allow for easy exchange of preconditioners for the whole system inside the CPRPreconditioner.

With this patch to change the preconditioner for the whole system if more easy now. The default stays the same as before, ILU(0).
I see now, Preconditioner applies to the whole system, EllipticPreconditioner to the part. To make this clearer (in case I am not the only slow one...) I think it should be called WholeSystemPreconditioner or something like that, to be more precise.