[FEM] Elmer: fix small number property handling

- use an expression as discussed: https://forum.freecadweb.org/viewtopic.php?p=613897#p613897

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

@@ -105,7 +105,11 @@ class Proxy(equation.Proxy):
             "Linear System",
             ""
         )
-        obj.LinearTolerance = 1e-8
+        # we must set an expression because we don't have a UI, the user has to
+        # view and edit the tolerance via the property editor and this does not
+        # yet allow to view and edit small numbers in scientific notation
+        # forum thread: https://forum.freecadweb.org/viewtopic.php?p=613897#p613897
+        obj.setExpression('LinearTolerance', "1e-8")
         obj.addProperty(
             "App::PropertyInteger",
             "LinearIterations",
@@ -119,7 +123,8 @@ class Proxy(equation.Proxy):
             "Steady State",
             ""
         )
-        obj.SteadyStateTolerance = 1e-5
+        # same as with LinearTolerance
+        obj.setExpression('SteadyStateTolerance', "1e-5")
         obj.addProperty(
             "App::PropertyBool",
             "Stabilize",

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

@@ -70,11 +70,15 @@ class Proxy(linear.Proxy):
             "Nonlinear System",
             ""
         )
-        obj.NonlinearTolerance = 1e-8
         obj.NonlinearIterations = 500
-        obj.RelaxationFactor = 1
         obj.NonlinearNewtonAfterIterations = 3
-        obj.NonlinearNewtonAfterTolerance = 1e-3
+        # for small numbers we must set an expression because we don't have a UI,
+        # the user has to view and edit the tolerance via the property editor and
+        # this does not yet allow to view and edit small numbers in scientific notation
+        # forum thread: https://forum.freecadweb.org/viewtopic.php?p=613897#p613897
+        obj.setExpression('NonlinearTolerance', "1e-8")
+        obj.setExpression('NonlinearNewtonAfterTolerance', "1e-3")
+        obj.setExpression('RelaxationFactor', "1.0") # must often be < 1 down to 0.01
 
 
 class ViewProxy(linear.ViewProxy):