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:

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");

dune/common/test/parameterizedobjecttest.cc

@@ -44,15 +44,15 @@ int main()
     CheckInstance(FactoryA, Ai, 0);
     CheckInstance(FactoryA, Bi, 1);
 
-    // default parameter
+    // default constructor
     using FactoryAd = Dune::ParameterizedObjectFactory<InterfaceA>;
-    FactoryAd::define<Ad>("Ad");
-    FactoryAd::define<Bd>("Bd");
+    FactoryAd::define<Ax>("Ax");
+    FactoryAd::define<Bx>("Bx");
     Dune::ParameterTree param;
-    CheckInstance(FactoryAd, Ad, param);
-    CheckInstance(FactoryAd, Bd, param);
+    CheckInstance(FactoryAd, Ax);
+    CheckInstance(FactoryAd, Bx);
 
-    // no parameter
+    // explicitly request the default constructor
     using FactoryAx = Dune::ParameterizedObjectFactory<InterfaceA()>;
     FactoryAx::define<Ax>("Ax");
     FactoryAx::define<Bx>("Bx");