move a few more classes into the Dune::Solvers namespace

For me "anything unpleasant" e.g. means any class with public member variables. Theses are very prone for user errors and a real pain to maintain. I strongly support to only move stuff that has been cleaned up and carefully reviewed to the namespace.

@burchardt_at_igpm.rwth-aachen.de Would you mind if I partially reverted your commit for the discussed reason?

@maxka: No, I think SolverResult, numproc.hh and solver.hh are only things I use indirectly. So I don't mind where they are.

AMGStep exhibits the public member variables SmootherArgs and Criterion and therefore falls into @graeser 's unwanted list. @burchardt_at_igpm.rwth-aachen.de how do you feel about reverting this as well?

The protected members in EnergyNorm seem to be fine (to me).

Or how about we make these members private and add setters and getters?