[bugfix] Restore reference semantics of ReferenceElement methods

By using decltype(auto) we forward references of the
implementation as references. This reverts the interface
change introduced in !52 in case the implementation returns
references. As a consequence the all tests in dune-localfunctions
pass again.

For (possible) implementations returning values these will
also be forwarded. In the future (after a grace period of
one release), this should be switched to references completely
as proposed in !52.

dune/geometry/referenceelement.hh

@@ -140,14 +140,14 @@ namespace Dune {
        *  \param[in]  i      number of subentity E (0 <= i < size( c ))
        *  \param[in]  c      codimension of subentity E
        */
-      GeometryType type(int i, int c) const
+      decltype(auto) type(int i, int c) const
       {
         return _impl->type(i,c);
       }
 
 
       /** \brief obtain the type of this reference element */
-      GeometryType type() const
+      decltype(auto) type() const
       {
         return _impl->type();
       }
@@ -162,7 +162,7 @@ namespace Dune {
        *  \param[in]  i   number of subentity E (0 <= i < size( c ))
        *  \param[in]  c   codimension of subentity E
        */
-      Coordinate position(int i, int c) const
+      decltype(auto) position(int i, int c) const
       {
         return _impl->position(i,c);
       }
@@ -213,7 +213,7 @@ namespace Dune {
        *
        *  \param[in]  face  index of the face, whose normal is desired
        */
-      Coordinate integrationOuterNormal(int face) const
+      decltype(auto) integrationOuterNormal(int face) const
       {
         return _impl->integrationOuterNormal(face);
       }