[FEM] Elmer: set sensible defaults for elasticity equation

- for BiCGstabl the minimal degree is 2, but 3 is recommended
- when adding the equation, it should do something, therefore enable the calculation of the principal stresses (as it was, nothing was calculated except of the displacement)

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

@@ -82,6 +82,7 @@ class Proxy(linear.Proxy, equationbase.ElasticityProxy):
         )
         obj.EigenmodesCount = 5
         obj.Priority = 10
+        obj.CalculatePrincipal = True
 
 
 class ViewProxy(linear.ViewProxy, equationbase.ElasticityViewProxy):

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

@@ -91,6 +91,7 @@ class Proxy(equation.Proxy):
             "Linear System",
             ""
         )
+        obj.BiCGstablDegree = 3
         obj.addProperty(
             "App::PropertyEnumeration",
             "LinearPreconditioning",