Create IFC_roundtrip_specifications.md

IFC_roundtrip_specifications.md

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+This file lists "exercises" to be performed by a BIM application to achieve adequate round-tripping 
+with IFC files. The application should successfully complete all the steps.
+
+## 1. importing and exporting a simple extrusion
+
+The application should be able to export and import an IFC file containing one IfcBuildingElementProxy 
+entity, with one representation, which is an IfcExtrudedAreaSolid, based on an IfcArbitraryClosedProfileDef
+made of an IfcPolyline, like the example below:
+
+```
+  #20= IFCBUILDINGELEMENTPROXY('0ohBfsArr3ruXYxacT4yl5',#1,'NOTDEFINED',$,$,#2,#21,$,.NOTDEFINED.);
+    #21= IFCPRODUCTDEFINITIONSHAPE($,$,(#22));
+      #22= IFCSHAPEREPRESENTATION(#9,'Body','SweptSolid',(#23));
+        #23= IFCEXTRUDEDAREASOLID(#24,$,#25,2000.);
+          #24 = IFCARBITRARYCLOSEDPROFILEDEF(.AREA., $, #26);
+            #26 = IFCPOLYLINE((#27, #28, #29, #30));
+              #27 = IFCCARTESIANPOINT((0., 0.));
+              #28 = IFCCARTESIANPOINT((1000., 0.));
+              #29 = IFCCARTESIANPOINT((1000., 1000.));
+              #30 = IFCCARTESIANPOINT((0., 1000.));
+          #25= IFCDIRECTION((0.,0.,1.));
+```
+
+#### Import criteria
+
+* The extrusion can be changed after import
+* The base polyline can be edited after import
+
+#### Export criteria
+
+* The exported IFC file contains an IfcBuildingElementProxy, with an IfcExtrudedAreaSolid as its representation
+and an IfcArbitraryClosedProfileDef made of an IfcPolyline as its profile
+
+#### Results
+
+|                | BlenderBIM | FreeCAD | Revit    | ArchiCAD | BricsCAD |
+| -------------- | ---------- | ------- | -------- | -------- | -------- |
+| passed?        |            |         |          |          |          |
+| exported file: |            |         |          |          |          |
+| imported file: |            |         |          |          |          |