Improve IFC export (structural analysis) for slabs

I did my best (I'm a programming novice) to understand the code and to improve the IFC export for slabs.
ATM for every slab all contour edges are exported as IFCSTRUCTURALCURVEMEMBER except the last closing one (see forum thread https://forum.freecadweb.org/viewtopic.php?f=39&t=54286).
This changes aim to also add the last closing edge for slabs.

src/Mod/Arch/exportIFCStructuralTools.py

@@ -105,17 +105,28 @@ def createStructuralMember(ifcfile,ifcbin,obj):
     uid = ifcopenshell.guid.new
     owh = ifcfile.by_type("IfcOwnerHistory")[0]
     ctx = getStructuralContext(ifcfile)
+
+    # find edges to convert into structural members
     edges = None
     if Draft.getType(obj) not in ["Structure"]:
+        # for non structural elements 
         if ALLOW_LINEAR_OBJECTS and obj.isDerivedFrom("Part::Feature"):
+            # for objects created with Part workbench
             if obj.Shape.Faces:
+                # for objects with faces
                 return None
             elif not obj.Shape.Edges:
+                # for objects without edges
                 return None
             else:
                 edges = obj.Shape.Edges
     else:
-        wire = Part.makePolygon([obj.Placement.multVec(n) for n in obj.Nodes])
+        # for structural elements with nodes
+        nodes = [obj.Placement.multVec(n) for n in obj.Nodes]
+        if len(nodes) > 2:
+            # when there are more then 2 nodes (i.e. for slabs) append closing node to produce closing edge
+            nodes.append(nodes[0])
+        wire = Part.makePolygon(nodes)
         edges = wire.Edges
     if not edges:
         return None