Warn when SubgridL2FunctionalAssembler falls back to L2FunctionalAssembler.

dune/fufem/assemblers/localassemblers/subgridl2functionalassembler.hh

@@ -102,6 +102,7 @@ class SubgridL2FunctionalAssembler :
             if (not(fAsHostGridFunction_))
             {
                 L2FunctionalAssembler<GridType,TrialLocalFE,T>::assemble(element, localVector, tFE);
+                Dune::dwarn << "SubgridL2FunctionalAssembler fallback to L2FunctionalAssembler. Make sure the given QuadKey is appropriate." << std::endl;
                 return;
             }