Follow-up from "Added first version of generic factory using istl solver factory."

The following discussion from !1839 (closed) should be addressed:

@timok started a discussion: (+1 comment)

New classes in Dumux can't have TypeTag anymore. Exceptions are some convenience aliases or structs to bridge the time until a TypeTag dependency is fully resolved. We maybe could have


template <class Matrix, class Vector, class GridGeometry>
struct SolverTraits
{
    using AmgTraits = Dumux::AmgTraits<Matrix, Vector, GridGeometry>;
};

template <class GridView, class SolverTraits>
class GenericIstlSolverFactoryBackend : public LinearSolver
{...};


template<class TypeTag>
using IstlSolverFactoryBackend = GenericIstlSolverFactoryBackend<Property stuff like for AMGBackend>