Feature/proximal newton solver

Introduce a Riemannian Proximal-Newton Solver as an alternative for the Trust-Region Solver. Also add 3 cleanup commits.

dune/gfe/riemanniantrsolver.cc

@@ -470,7 +470,7 @@ void RiemannianTrustRegionSolver<Basis,TargetSpace>::solve()
             if (mgStep && solved)
                 corr_global = mgStep->getSol();
 
-            //std::cout << "Correction: " << std::endl << corr_global << std::endl;
+            std::cout << "Two norm of the correction: " << corr_global.two_norm() << std::endl;
         }
 
         // Distribute solution
@@ -479,6 +479,7 @@ void RiemannianTrustRegionSolver<Basis,TargetSpace>::solve()
 
 #if HAVE_MPI
         corr = vectorComm.scatter(corr_global);
+        solved = grid_->comm().min(solved);
 #else
         corr = corr_global;
 #endif