[FEM] add current density constraint

- also add corresponding App::PropertyCurrentDensity

- also fix a typo in test_object.py

src/App/Application.cpp

@@ -1914,6 +1914,7 @@ void Application::initTypes()
     App::PropertyAcceleration       ::init();
     App::PropertyForce              ::init();
     App::PropertyPressure           ::init();
+    App::PropertyCurrentDensity     ::init();
     App::PropertyElectricPotential  ::init();
     App::PropertyVacuumPermittivity ::init();
     App::PropertyInteger            ::init();