Fixes towards allowing stateful bases in TrustRegionSolver

Currently the TrustRegionSolver class default-constructs a few bases
objects of its own.  That only works if the actual bases used for
the simulation does not contain state, because the state doesn't
make it into the new bases.

Fix this by avoid the default constructor.

There are remaining issues for the HAVE_DUNE_PARMG case, which
I indent to ignore for now.

dune/elasticity/common/trustregionsolver.cc

@@ -90,7 +90,7 @@ setup(const typename BasisType::GridView::Grid& grid,
     std::conditional_t< // do we have a dune-functions basis?
       Dune::models<Dune::Functions::Concept::GlobalBasis<GridView>, BasisType>(),
         BasisType,
-        DuneFunctionsBasis<BasisType> > basis(grid.leafGridView());
+        DuneFunctionsBasis<BasisType> > basis(assembler_->basis_);
     // ////////////////////////////////
     //   Create a multigrid solver
     // ////////////////////////////////
@@ -361,14 +361,11 @@ void TrustRegionSolver<BasisType,VectorType>::solve()
         mgStep = dynamic_cast<MonotoneMGStep<MatrixType,CorrectionType>*>(&iterationStep);
 
     }
-
-    BasisType basis(grid_->leafGridView());
 #else
-    BasisType basis(grid_->levelGridView(grid_->maxLevel()));
     BasisType coarseBasis(grid_->levelGridView(0));
     std::vector<BoxConstraint<typename VectorType::field_type, blocksize>> coarseTrustRegionObstacles(coarseBasis.size());
 #endif
-    MaxNormTrustRegion<blocksize> trustRegion(basis.size(), initialTrustRegionRadius_);
+    MaxNormTrustRegion<blocksize> trustRegion(assembler_->basis_.size(), initialTrustRegionRadius_);
 
     std::vector<BoxConstraint<field_type,blocksize> > trustRegionObstacles;