FEM: use python base constraint class, result mechanical

src/Mod/Fem/femobjects/_FemResultMechanical.py

@@ -27,14 +27,17 @@ __url__ = "http://www.freecadweb.org"
 #  \ingroup FEM
 #  \brief FreeCAD DocumentObject class to hold mechanical results in FEM workbench
 
+from . import FemConstraint
 
-class _FemResultMechanical():
+
+class _FemResultMechanical(FemConstraint.Proxy):
     """The Fem::_FemResultMechanical's Proxy python type, add result specific properties
     """
+
+    Type = "Fem::FemResultMechanical"
+
     def __init__(self, obj):
-        self.Type = "Fem::FemResultMechanical"
-        self.Object = obj  # keep a ref to the DocObj for nonGui usage
-        obj.Proxy = self  # link between App::DocumentObject to this object
+        super(_FemResultMechanical, self).__init__(obj)
 
         obj.addProperty(
             "App::PropertyString",