[App] add property for electric potentials

- in preparation to fix the electrostatic simulation mess

src/App/PropertyUnits.cpp

@@ -339,6 +339,18 @@ PropertyForce::PropertyForce()
     setUnit(Base::Unit::Force);
 }
 
+//**************************************************************************
+//**************************************************************************
+// PropertyElectricPotential
+//++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+
+TYPESYSTEM_SOURCE(App::PropertyElectricPotential, App::PropertyQuantity)
+
+PropertyElectricPotential::PropertyElectricPotential()
+{
+    setUnit(Base::Unit::ElectricPotential);
+}
+
 //**************************************************************************
 //**************************************************************************
 // PropertyVacuumPermittivity