Fem: Add frd format converter to VTK

src/Mod/Fem/App/FemVTKTools.h

@@ -71,6 +71,8 @@ public:
 
     // write FemResult (activeObject if res= NULL) to vtkUnstructuredGrid dataset file
     static void writeResult(const char* filename, const App::DocumentObject* res = nullptr);
+
+    static void frdToVTK(const char* filename);
 };
 }  // namespace Fem