Simplification: merge the two classes that implement element facets in 3d into one

Previously, the implementation of UGGrid entities for facets of 3d elements
consisted of two classes: UGGridEntity<1,3>, a specialization of the generic
implementation class for UGGrid entities, and UGFaceEntity, a special
implementation class for the codim-1 case in 3d.  There was never a real
reason for this separation into two classes, and this patch does away
with it.  This leads to a minor code simplification.

Note: The same separation into two classes holds for grid edges, and there is
a reason.  The UGEdgeEntity class, which contains the actual implementation,
serves for *two* cases: UGGridEntity<1,2> and UGGridEntity<2,3>.
C++ partial specialization rules apparently do not allow to merge all of this
into one class.

dune/grid/uggrid/uggridentity.hh

@@ -400,13 +400,14 @@ namespace Dune {
   };
 
 
-  /*! \brief UGGrid face entity
+  /*! \brief UGGrid face entity in 3d grids
    * \ingroup UGGrid
    */
-  template<int dim, class GridImp>
-  class UGFaceEntity
+  template<class GridImp>
+  class UGGridEntity<1,3,GridImp>
   {
     enum {codim = 1};
+    enum {dim = 3};
     typedef typename GridImp::ctype UGCtype;
 
     typedef typename GridImp::Traits::template Codim<codim>::GeometryImpl GeometryImpl;
@@ -415,12 +416,12 @@ namespace Dune {
 
     typedef typename GridImp::template Codim<codim>::Geometry Geometry;
 
-    UGFaceEntity()
+    UGGridEntity()
       : target_(nullptr)
       , gridImp_(nullptr)
     {}
 
-    UGFaceEntity(typename UG_NS<dim>::template Entity<codim>::T* target, const GridImp* gridImp)
+    UGGridEntity(typename UG_NS<dim>::template Entity<codim>::T* target, const GridImp* gridImp)
     {
       setToTarget(target,gridImp);
     }
@@ -525,7 +526,7 @@ namespace Dune {
     }
 
     //! equality
-    bool equals(const UGFaceEntity& other) const
+    bool equals(const UGGridEntity& other) const
     {
       return getTarget() == other.getTarget();
     }
@@ -542,25 +543,6 @@ protected:
     const GridImp* gridImp_;
   };
 
-  /*! \brief Specialization for faces in 3D
-   * \ingroup UGGrid
-   */
-  template<class GridImp>
-  class UGGridEntity<1,3,GridImp>
-    : public UGFaceEntity<3,GridImp>
-  {
-
-  public:
-
-    UGGridEntity()
-    {}
-
-    UGGridEntity(typename UG_NS<3>::template Entity<1>::T* target, const GridImp* gridImp)
-      : UGFaceEntity<3,GridImp>(target,gridImp)
-    {}
-
-  };
-
 
 
   //***********************