istl AMG preconditioner added

The dune-istl AMG method is available as precondition. The corresponding initfile parameters are

[...]->solver->precon: [amg,fastamg]
[...]->solver->precon->smoother: [jacobi,sor,ssor] % later more smoothers can be added

[...]->solver->precon->[...] % general amg parameters, see Dune::Amg::Parameters
[...]->solver->precon->coarsening->[...] % Parameters of Dune::Amg::CoarseningParameters
[...]->solver->precon->aggregation->[...] % Parameters of Dune::Amg::AggregationParameters
[...]->solver->precon->dependency->[...] % Parameters of Dune::Amg::DependencyParameters
[...]->solver->precon->smoother->[...] % Parameters of Dune::Amg::DefaultSmootherArgs

See amdis/linearalgebra/istl/AMGPrecon.hpp for a detailed list of parameters. The parameter names follow the variable names in the Dune Parameters class.

merged

mentioned in commit b0713779