enabled DUNE-ISTL::AMG and DUNE-ISTL::CGSolver in CPRPreconditioner.

The default behavior is the same as before, i.e. DUNE-ISTL::ILU0 and DUNE-ISTL::BiCGSolver are used 
for solving the elliptic problem. 

For some setting the AMG/CG combination can really speed up the solver, for others it doesn't. I still need to investigate more on the solver part. Maybe Markus can have  a look on the AMG args.
Just an idea: could it be advantageous to keep the previous solution (only set to zero in constructor), as a starting point for the iterations?