Fixed tests

interface/HUTN/test/modelling_language/code/my_petrinet_with_MM.mvc

@@ -1,7 +1,7 @@
 import models/SimpleClassDiagrams as SCD
 
 SCD PetriNets{
-    Class Natural {}
+    SimpleAttribute Natural {}
     Class Place{
         tokens : Natural
     }

interface/HUTN/test/modelling_language/code/petrinets.mvc

@@ -1,7 +1,7 @@
 import models/SimpleClassDiagrams as SCD
 
 SCD PetriNets{
-    Class Natural {}
+    SimpleAttribute Natural {}
     Class Place{
         tokens : Natural
     }

interface/HUTN/test/modelling_language/code/petrinets_constraints.mvc

@@ -2,15 +2,16 @@ import models/SimpleClassDiagrams as SCD
 include "primitives.alh"
 
 SCD PetriNets{
-    Class Natural {
-        $
-            if (bool_not(is_physical_int(self))):
-                return "Natural has no integer value"!
-            elif (integer_lt(self, 0)):
-                return "Natural does not have a positive or zero value"!
-            else:
-                return "OK"!
-         $
+    SimpleAttribute Natural {
+        constraint = $
+            String function constraint(model : Element, name : String):
+                if (bool_not(is_physical_int(model["model"][name]))):
+                    return "Natural has no integer value"!
+                elif (integer_lt(model["model"][name], 0)):
+                    return "Natural does not have a positive or zero value"!
+                else:
+                    return "OK"!
+            $
     }
     Class Place{
         tokens : Natural

interface/HUTN/test/modelling_language/code/simpleclassdiagrams.mvc

@@ -2,8 +2,8 @@ import /formalisms/SimpleClassDiagrams as SCD
 
 SCD SimpleClassDiagrams{
     Class Any {}
-    Class Natural {}
-    Class String {}
+    SimpleAttribute Natural {}
+    SimpleAttribute String {}
     Class Class{
         lower_cardinality : Natural
         upper_cardinality : Natural