New system module with UI to browse systems in an IFC.

src/ifcopenshell-python/ifcopenshell/api/system/assign_system.py

@@ -0,0 +1,27 @@
+import ifcopenshell
+import ifcopenshell.api
+
+
+class Usecase:
+    def __init__(self, file, **settings):
+        self.file = file
+        self.settings = {
+            "product": None,
+            "system": None,
+        }
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        if not self.settings["system"].IsGroupedBy:
+            return self.file.create_entity("IfcRelAssignsToGroup", **{
+                "GlobalId": ifcopenshell.guid.new(),
+                "OwnerHistory": ifcopenshell.api.run("owner.create_owner_history", self.file),
+                "RelatedObjects": [self.settings["product"]],
+                "RelatingGroup": self.settings["system"]
+            })
+        rel = self.settings["system"].IsGroupedBy[0]
+        related_objects = set(rel.RelatedObjects) or set()
+        related_objects.add(self.settings["product"])
+        rel.RelatedObjects = list(related_objects)
+        ifcopenshell.api.run("owner.update_owner_history", self.file, **{"element": rel})