[!343] Avoid clang7 -Wundefined-var-template for UGGrid::heapSize_

Merge branch 'uggrid-heapSize-Wundefined-var-template' into 'master'

ref:core/dune-grid Closes: [#91]

WIP:

-   [x] Manually verify in job trace the warning is gone (the remaining
    instances of the same error seem Alberta-related)

See merge request [!343]

  [#91]: gitlab.dune-project.org/NoneNone/issues/91
  [!343]: gitlab.dune-project.org/core/dune-grid/merge_requests/343


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