[FEM] potential constraint: change default value

- in practice one wants to apply potentials to faces via this constraint, therefore enable to set a potential by default, also assure that the initial unit is V

src/Mod/Fem/femobjects/constraint_electrostaticpotential.py

@@ -52,7 +52,9 @@ class ConstraintElectrostaticPotential(base_fempythonobject.BaseFemPythonObject)
                 "Parameter",
                 "Electric Potential"
             )
-            obj.Potential = 0.0
+            # setting  1 V assures that the unit does not switch to mV
+            # and the constraint holds usually Volts
+            obj.Potential = "1 V"
 
         if not hasattr(obj, "PotentialEnabled"):
             obj.addProperty(
@@ -61,7 +63,7 @@ class ConstraintElectrostaticPotential(base_fempythonobject.BaseFemPythonObject)
                 "Parameter",
                 "Potential Enabled"
             )
-            obj.PotentialEnabled = False
+            obj.PotentialEnabled = True
 
         if not hasattr(obj, "PotentialConstant"):
             obj.addProperty(