Merge branch 'feature/parameterizedobjectfactory' into 'master'

Feature/parameterizedobjectfactory

the idea is to have a generic factory which can be extended by the user at run time and maps a key (usually an `std::string`) to a constructor-call (usually taking a `Dune::ParameterTree`).

We want something like a plugin-system, to instantiate implementations of an abstract interface. Usually a factory can only create a hard-coded list of objects. The proposed ParameterizedObjectFactory different modules (or the user) can register implementations of the interface, without modifying the factory code.

Example:
```C++
using V = Dune::BlockVector<Dune::FieldVector<double,2>>;
using M = Dune::BCRSMatrix<Dune::FieldMatrix<double,2,2>>;

using PreconditionerFactory = 
  Dune::ParameterizedObjectFactory<Dune::Preconditioner<V,V>(const M&, const Dune::ParameterTree&)>;

PreconditionerFactory::define<Dune::SeqGS<M,V,V>>("Gauss-Seidel");
```


See merge request !3