[bugfix] Inititalize local matrix/vector correctly.

This should have been done earlier, but only became
visible when switching to the dunefunctions assemblers
where the local matrix/vector is no longer instantiated
separately for every element.

dune/fufem/assemblers/localassemblers/scaledsumfunctionalassembler.hh

@@ -44,6 +44,7 @@ public:
    */
   virtual void assemble(const Element& element, LocalVector& localVector,
                         const FE& tFE) const {
+    localVector = 0.0;
     for (auto&& scaledAssembler : scaledAssemblers_) {
       LocalVector v(localVector.size());
       scaledAssembler.second->assemble(element, v = 0.0, tFE);

dune/fufem/assemblers/localassemblers/scaledsumoperatorassembler.hh

@@ -63,6 +63,7 @@ public:
   virtual void assemble(const Element& element, LocalMatrix& localMatrix,
                         const TrialLocalFE& tFE,
                         const AnsatzLocalFE& aFE) const {
+    localMatrix = 0.0;
     for (auto&& scaledAssembler : scaledAssemblers_) {
       LocalMatrix m(localMatrix.N(), localMatrix.M());
       m = 0.0;