[course][exercise properties] added todo in neumann for consistency

exercises/exercise-properties/problem.hh

@@ -176,6 +176,7 @@ public:
      */
     NumEqVector neumannAtPos(const GlobalPosition &globalPos) const
     {
+// TODO: reformulate the neumann boundary condition's values in terms of volume instead of mass injected per meter and second
         NumEqVector values(0.0);
         if (onInlet_(globalPos))
             values[contiDNAPLEqIdx] = -0.04; // kg / (m * s)