[FEM] Elmer: set constraints for relaxation factor

- the relaxation factor is crucial for nonlinear systems thus help the user by setting constraints and convenient steps.
Since the property stays a float no update is necessary, existing properties just stay a normal float

src/Mod/Fem/femsolver/elmer/equations/nonlinear.py

@@ -66,7 +66,7 @@ class Proxy(linear.Proxy):
             ""
         )
         obj.addProperty(
-            "App::PropertyFloat",
+            "App::PropertyFloatConstraint",
             "RelaxationFactor",
             "Nonlinear System",
             (
@@ -83,7 +83,7 @@ class Proxy(linear.Proxy):
         # forum thread: https://forum.freecadweb.org/viewtopic.php?p=613897#p613897
         obj.setExpression("NonlinearTolerance", "1e-7")
         obj.setExpression("NonlinearNewtonAfterTolerance", "1e-3")
-        obj.RelaxationFactor = 1.0
+        obj.RelaxationFactor = (1.0, 0.01, 2.0, 0.1)
 
 
 class ViewProxy(linear.ViewProxy):