[WIP] Feature/solverfactory

This MR mainly contains the factory implementation from !5 (closed) with some cleanup:

• Allow creating solvers / preconditioners from ParameterTrees.

• A solver (including preconditioner) can now be configured via an ini file like this:

[solver]
precond = SeqSSOR
solver = CGSolver

[SeqSSOR]
iterations = 1
relaxation = 1.8

[CGSolver]
reduction=1e-9
maxit = 5000
verbose = 3

• The corresponding dune code currently looks like this:

auto solver = SolverPrecondFactory::create<VectorType> (matrix, configuration, "solver");

• Both the actual type of the solver/preconditioner and their parameters can chosen in the ini file.
• It is also possible to create only a solver or only a preconditioner this way. See the factory doxygen for details.

This MR depends on !84 (merged).

Note: change target branch to master, once !84 (merged) is merged!

dune/istl/paamg/kamg.hh

@@ -35,9 +35,10 @@ namespace Dune
       typedef typename AMG::Range Range;
     public:
 
-      enum {
-        /** @brief The solver category. */
-        category = AMG::category
+      //! Category of the preconditioner (see SolverCategory::Category)
+      virtual SolverCategory::Category category() const
+      {
+        return amg_.category();
       };
 
       /**
@@ -163,10 +164,12 @@ namespace Dune
       /** @brief The type of the scalar product. */
       typedef typename Amg::ScalarProduct ScalarProduct;
 
-      enum {
-        /** @brief The solver category. */
-        category = Amg::category
+      //! Category of the preconditioner (see SolverCategory::Category)
+      virtual SolverCategory::Category category() const
+      {
+        return amg.category();
       };
+
       /**
        * @brief Construct a new amg with a specific coarse solver.
        * @param matrices The already set up matix hierarchy.
@@ -338,7 +341,7 @@ namespace Dune
 
       if(matrix!=amg.matrices_->matrices().finest())
         while(true) {
-          scalarproducts.push_back(std::shared_ptr<typename Amg::ScalarProduct>(Amg::ScalarProductChooser::construct(*pinfo)));
+          scalarproducts.push_back(createScalarProduct<X>(*pinfo,category()));
           std::shared_ptr<InverseOperator<Domain,Range> > ks =
             std::shared_ptr<InverseOperator<Domain,Range> >(new KrylovSolver(*matrix, *(scalarproducts.back()),
                                                                         *(ksolvers.back()), levelDefectReduction,