[mpfa][elemvolvars] reserve memory only once

Until now, the private vector of volume variables was resized and
afterwards additional memory was reserved for the boundary vol vars.
This can cause reallocation to happen. In order to avoid this we now
reserve memory once and push back the vol vars that are created.

dumux/discretization/cellcentered/mpfa/elementvolumevariables.hh

@@ -130,39 +130,38 @@ public:
         const auto numVolVars = assemblyMapI.size() + 1;
 
         // resize local containers to the required size (for internal elements)
-        volumeVariables_.resize(numVolVars);
-        volVarIndices_.resize(numVolVars);
-        unsigned int localIdx = 0;
+        const auto maxNumBoundaryVolVars = maxNumBoundaryVolVars_(fvGeometry);
+        volumeVariables_.reserve(numVolVars+maxNumBoundaryVolVars);
+        volVarIndices_.reserve(numVolVars+maxNumBoundaryVolVars);
 
-        // update the volume variables of the element at hand
+        VolumeVariables volVars;
         const auto& scvI = fvGeometry.scv(globalI);
-        volumeVariables_[localIdx].update(elementSolution(element, sol, fvGridGeometry),
-                                          problem,
-                                          element,
-                                          scvI);
-        volVarIndices_[localIdx] = scvI.dofIndex();
-        ++localIdx;
+        volVars.update(elementSolution(element, sol, fvGridGeometry),
+                       problem,
+                       element,
+                       scvI);
+
+        volVarIndices_.push_back(scvI.dofIndex());
+        volumeVariables_.emplace_back(std::move(volVars));
 
         // Update the volume variables of the neighboring elements
         for (auto&& dataJ : assemblyMapI)
         {
             const auto& elementJ = fvGridGeometry.element(dataJ.globalJ);
             const auto& scvJ = fvGeometry.scv(dataJ.globalJ);
-            volumeVariables_[localIdx].update(elementSolution(elementJ, sol, fvGridGeometry),
-                                              problem,
-                                              elementJ,
-                                              scvJ);
-            volVarIndices_[localIdx] = scvJ.dofIndex();
-            ++localIdx;
+            VolumeVariables volVarsJ;
+            volVarsJ.update(elementSolution(elementJ, sol, fvGridGeometry),
+                            problem,
+                            elementJ,
+                            scvJ);
+
+            volVarIndices_.push_back(scvJ.dofIndex());
+            volumeVariables_.emplace_back(std::move(volVarsJ));
         }
 
         // maybe prepare boundary volume variables
-        const auto maxNumBoundaryVolVars = maxNumBoundaryVolVars_(fvGeometry);
         if (maxNumBoundaryVolVars > 0)
         {
-            volumeVariables_.reserve(numVolVars+maxNumBoundaryVolVars);
-            volVarIndices_.reserve(numVolVars+maxNumBoundaryVolVars);
-
             if (fvGeometry.hasBoundaryScvf())
             {
                 // treat the BCs inside the element