[exercise][model] Make sure task 1-5 can be done without imageIntensity interface

exercises/exercise-model/main.cc

@@ -161,7 +161,7 @@ int main(int argc, char** argv)
 
     // We initialize the VTK output module and write out the initial concentration field
     // VtkOutputModule<GridVariables, SolutionVector> vtkWriter(*gridVariables, sol, problem->name());
-    // vtkWriter.addVolumeVariable([](const auto& vv){ return vv.imageIntensity(); }, "imageIntensity");
+    // vtkWriter.addVolumeVariable([](const auto& vv){ return vv.priVar(0); }, "imageIntensity");
     // vtkWriter.write(0.0);
 
     // We instantiate time loop using start and end time as well as