Avoid reallocation of local matrix

Resize local matrix once to `localView.maxSize()`. Then
one does not need to resize is on each element. In fact
each call to `Dune::Matrix::setSize()` leads to a reallocation,
even if the size does not change.

There's an issue with a potential corner case: If a local assembler
assumes, that the local matrix has exactly the `localView.size()`
this will now fail.

dune/fufem/assemblers/dunefunctionsoperatorassembler.hh

@@ -131,12 +131,13 @@ public:
 
     auto localMatrix = LocalMatrix();
 
+    localMatrix.setSize(trialLocalView.maxSize(), ansatzLocalView.maxSize());
+
     for (const auto& element : elements(trialBasis_.gridView()))
     {
       // bind the localViews to the element
       bind(ansatzLocalView, trialLocalView, element);
 
-      localMatrix.setSize(trialLocalView.size(), ansatzLocalView.size());
       localAssembler(element, localMatrix, trialLocalView, ansatzLocalView);
 
       // Add element stiffness matrix onto the global stiffness matrix
@@ -179,15 +180,17 @@ public:
     using MatrixContainer = Dune::Matrix<LocalMatrix>;
     auto matrixContrainer = MatrixContainer(2,2);
 
+    matrixContrainer[0][0].setSize(insideTrialLocalView.maxSize(),  insideAnsatzLocalView.maxSize());
+    matrixContrainer[0][1].setSize(insideTrialLocalView.maxSize(),  outsideAnsatzLocalView.maxSize());
+    matrixContrainer[1][0].setSize(outsideTrialLocalView.maxSize(), insideAnsatzLocalView.maxSize());
+    matrixContrainer[1][1].setSize(outsideTrialLocalView.maxSize(), outsideAnsatzLocalView.maxSize());
+
     for (const auto& element : elements(trialBasis_.gridView()))
     {
       insideTrialLocalView.bind(element);
 
       insideAnsatzLocalView.bind(element);
 
-      // Resize
-      matrixContrainer[0][0].setSize(insideTrialLocalView.size(), insideAnsatzLocalView.size());
-
       for (const auto& is : intersections(trialBasis_.gridView(), element))
       {
 
@@ -217,10 +220,6 @@ public:
 
           outsideAnsatzLocalView.bind(outsideElement);
 
-          matrixContrainer[0][1].setSize(insideTrialLocalView.size(), outsideAnsatzLocalView.size());
-          matrixContrainer[1][0].setSize(outsideTrialLocalView.size(), insideAnsatzLocalView.size());
-          matrixContrainer[1][1].setSize(outsideTrialLocalView.size(), outsideAnsatzLocalView.size());
-
           localAssembler(is, matrixContrainer, insideTrialLocalView, insideAnsatzLocalView, outsideTrialLocalView, outsideAnsatzLocalView);
 
           for (size_t i=0; i < insideTrialLocalView.size(); i++)