[FEM] adapt electrostatic examples to changed unit handling

- also a coding improvement

src/Mod/Fem/femexamples/equation_electrostatics_electricforce_elmer_nongui6.py

@@ -190,7 +190,7 @@ def setup(doc=None, solvertype="elmer"):
     name_pot1 = "ElectrostaticPotential1"
     con_elect_pot1 = ObjectsFem.makeConstraintElectrostaticPotential(doc, name_pot1)
     con_elect_pot1.References = [(geom_obj, "Face2")]
-    con_elect_pot1.Potential = 0.00
+    con_elect_pot1.Potential = "0 V"
     con_elect_pot1.CapacitanceBody = 1
     con_elect_pot1.CapacitanceBodyEnabled = True
     con_elect_pot1.PotentialEnabled = True
@@ -204,7 +204,7 @@ def setup(doc=None, solvertype="elmer"):
         (geom_obj, "Face5"),
         (geom_obj, "Face6"),
         (geom_obj, "Face11")]
-    con_elect_pot2.Potential = 1.00
+    con_elect_pot2.Potential = "1 V"
     con_elect_pot2.CapacitanceBody = 2
     con_elect_pot2.CapacitanceBodyEnabled = True
     con_elect_pot2.PotentialEnabled = True

src/Mod/Fem/femtaskpanels/task_constraint_electrostaticpotential.py

@@ -32,7 +32,6 @@ __url__ = "https://www.freecadweb.org"
 
 import FreeCAD
 import FreeCADGui
-from FreeCAD import Units
 from femguiutils import selection_widgets
 from femtools import femutils
 from femtools import membertools
@@ -120,7 +119,7 @@ class _TaskPanel(object):
             # we could run into a syntax error on getting the quantity
             potential = None
             try:
-                potential = Units.Quantity(self._paramWidget.potentialQSB.text())
+                potential = self._paramWidget.potentialQSB.property('value')
             except ValueError:
                 FreeCAD.Console.PrintMessage(
                     "Wrong input. Not recognised input: '{}' "