added forward declaration of DOFVector

src/amdis/gridfunctions/DiscreteFunction.hpp

@@ -16,6 +16,9 @@
 
 namespace AMDiS
 {
+  template <class GB, class VT>
+  class DOFVector;
+  
   /// \class DiscreteFunction
   /// \brief A view on a subspace of a \ref DOFVector
   /**