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).
Unfortunately, SeqILU0 and SeqILUn have different constructors. I'm not sure if SeqILU0 and SeqILUn (n=0) do exactly the same thing.