[parallel] Uncomment the parallel interface member functions

Communicate seems to be required by DefaultGridView. I'm not sure why this wasn't a problem before as DefaultGridView doesn't seem to have changed.

dune/foamgrid/foamgrid.hh

@@ -535,11 +535,18 @@ class FoamGrid :
         }
 
         /** \brief Distributes this grid over the available nodes in a distributed machine
-        *
-        * \param minlevel The coarsest grid level that gets distributed
-        * \param maxlevel does currently get ignored
         */
-        void loadBalance(int strategy, int minlevel, int depth, int maxlevel, int minelement) {}
+        template<class DataHandle>
+        void loadBalance(DataHandle& data)
+        {
+            loadBalance();
+        }
+
+        void loadBalance()
+        {
+            if (comm().size() > 1)
+                DUNE_THROW(Dune::NotImplemented, "Load balancing not implemented. Foamgrid does not run in parallel yet!");
+        }
 
         /** \brief The communication interface
         *  @param T: array class holding data associated with the entities