[App] add property for electric potentials

- in preparation to fix the electrostatic simulation mess

src/App/Application.cpp

@@ -1912,6 +1912,7 @@ void Application::initTypes()
     App ::PropertyAcceleration      ::init();
     App ::PropertyForce             ::init();
     App ::PropertyPressure          ::init();
+    App ::PropertyElectricPotential ::init();
     App ::PropertyVacuumPermittivity::init();
     App ::PropertyInteger           ::init();
     App ::PropertyIntegerConstraint ::init();