Draft: [feature][Grid] Add possibility to store user data pointers on the grid object.

@andreas.dedner: Surprisingly easy, yet effective. Also take a look at the corresponding branch in dune-fem.

dune/grid/common/CMakeLists.txt

@@ -27,7 +27,8 @@ set(HEADERS
   partitionset.hh
   rangegenerators.hh
   sizecache.hh
-  scsgmapper.hh)
+  scsgmapper.hh
+  userdata.hh)
 
 add_subdirectory(test)