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.
You seem to mix styles a little when it comes to if statements, brace placement etc. I prefer:
```
if (foo) { // space after if, for, while etc., spaces around foo is ok, but not required
    // do stuff, indent 4 spaces
} else {
    // always braces around if and else bodies, prefer starting brace on same line with if, for, else etc.
    // do other stuff
}
```
Of course these are style issues that do not matter for correctness, but we care about these things, because we care about the code!