[mpfa][elemfluxvarscache] reserve more memory for local caches

In adaptive grids it might be necessary to store more interaction volume
data. Memory reallocation must be avoided as the stored pointers lose
validity otherwise.

dumux/discretization/cellcentered/mpfa/elementfluxvariablescache.hh

@@ -165,13 +165,14 @@ public:
             for (auto scvfIdx : dataJ.scvfsJ)
                 globalScvfIndices_[i++] = scvfIdx;
 
-        // reserve memory estimate for interaction volumes and corresponding data
+        // Reserve memory (over-) estimate for interaction volumes and corresponding data.
+        // The overestimate doesn't hurt as we are not in a memory-limited configuration.
+        // We need to avoid reallocation because in the caches we store pointers to the data handles.
         const auto numIvEstimate = getNoInteractionVolumesEstimate_(element, assemblyMapI);
-        const auto maxBoundaryIv = element.subEntities(dim);
         primaryInteractionVolumes_.reserve(numIvEstimate);
-        secondaryInteractionVolumes_.reserve(maxBoundaryIv);
+        secondaryInteractionVolumes_.reserve(numIvEstimate);
         primaryIvDataHandles_.reserve(numIvEstimate);
-        secondaryIvDataHandles_.reserve(maxBoundaryIv);
+        secondaryIvDataHandles_.reserve(numIvEstimate);
 
         // helper class to fill flux variables caches
         FluxVariablesCacheFiller filler(problem);
@@ -285,12 +286,19 @@ private:
     template<class AssemblyMap>
     std::size_t getNoInteractionVolumesEstimate_(const Element& element, const AssemblyMap& assemblyMap)
     {
-        //! Get the mpfa method only once per simulation
-        static const MpfaMethods method = GET_PROP_VALUE(TypeTag, MpfaMethod);
-
         // TODO: uncomment as soon as methods are re-implemented
-        if (method == MpfaMethods::oMethod /*|| method == MpfaMethods::oMethodFps*/)
-            return element.subEntities(dim);
+        // if statements are optimized away by the compiler
+        if (GET_PROP_VALUE(TypeTag, MpfaMethod) == MpfaMethods::oMethod /*|| method == MpfaMethods::oMethodFps*/)
+        {
+            //! Reserve memory for the case of each facet having neighbors being 4 levels higher. Memory limitations
+            //! do not play an important role here as global caching is disabled. In the unlikely case you want
+            //! to use higher local differences in element levels set a higher value for the parameter below
+            //! in your input file (note that some grids might only support levels differences of one anyway)
+            static const unsigned int maxDiff = getParamFromGroup<unsigned int>(GET_PROP_VALUE(TypeTag, ModelParameterGroup),
+                                                                                "Grid.MaxLocalElementLevelDifference",
+                                                                                4);
+            return element.subEntities(dim)*maxDiff;
+        }
       /*  else if (method == MpfaMethods::lMethod)
         {
             std::size_t numInsideScvfs = MpfaHelper::getNumLocalScvfs(element.geometry().type());