Add `Implementation` type to `EntityIterator`

dune-fem was using this type previously made available by EntityPointer.

Note that the type is now always available as the DUNE_GRID_EXPERIMENTAL_GRID_EXTENSIONS has been deprecated.

See the discussion in !164 (merged)

dune/grid/common/entityiterator.hh

@@ -33,6 +33,7 @@ namespace Dune
 
   public:
     typedef typename Grid::template Codim< codim >::Entity Entity;
+    using Implementation = IteratorImp;
 
     /** \brief Type of the reference used when derefencing the Ptr */
     typedef typename std::conditional<