Export T as field_type

Solvers from dune-solvers expect the number type to be exported
as field_type.  This patch adds the missing export to RealTuple.

[[Imported from SVN: r9866]]

dune/gfe/realtuple.hh

@@ -18,6 +18,7 @@ class RealTuple
 public:
 
     typedef T ctype;
+    typedef T field_type;
 
     /** \brief The type used for global coordinates */
     typedef Dune::FieldVector<T,N> CoordinateType;