Avoid warning comparing unsigned and signed integer

Signed-off-by: Jonathan Youett <youett@math.fu-berlin.de>

dune/elasticity/utilities/stresswriter.hh

@@ -91,7 +91,7 @@ auto StressWriter<DisplacementField, Grid>::getStress(const Dune::ParameterTree&
     std::vector<size_t> neighboringElementsPerVertex(nodalStress.size(), 0);
 
     for (const auto& e : elements(grid.leafGridView()))
-      for (size_t j = 0; j < e.geometry().corners(); j++) {
+      for (int j = 0; j < e.geometry().corners(); j++) {
         auto vertexIndex = indexSet.subIndex(e, j, Grid::dimension);
         neighboringElementsPerVertex[vertexIndex]++;
         nodalStress[vertexIndex] += stress[indexSet.index(e)];