Avoid clang7 -Wundefined-var-template for UGGrid::heapSize_

Closes: #91

dune/grid/uggrid.hh

@@ -870,6 +870,10 @@ namespace Dune {
 
   }; // end Class UGGrid
 
+  // those are defined in uggrid/uggrid.cc
+  template<> unsigned int UGGrid<2>::heapSize_;
+  template<> unsigned int UGGrid<3>::heapSize_;
+
   namespace Capabilities
   {
     /** \struct hasEntity