[vtkusers] Determining Actor type

[David Gobbi]

The method "IsA()" is the VTK method for type checking.  See the
vtkObjectBase documentation for more info.

if (actor->IsA("vtkCubeAxisActor") { ... }

All VTK classes also have a casting method called SafeDownCast() that
should be used instead of dynamic_cast<>().

   David