Fix failing tests from refactor which I really should've done before the release

src/ifcopenshell-python/ifcopenshell/util/system.py

@@ -17,6 +17,34 @@
 # along with IfcOpenShell.  If not, see <http://www.gnu.org/licenses/>.
 
 
+group_types = {
+    "IfcZone": ("IfcZone", "IfcSpace", "IfcSpatialZone"),
+    "IfcBuiltSystem": (
+        "IfcBuiltElement",
+        "IfcFurnishingElement",
+        "IfcElementAssembly",
+        "IfcTransportElement",
+    ),
+    "IfcBuildingSystem": (
+        "IfcBuildingElement",
+        "IfcFurnishingElement",
+        "IfcElementAssembly",
+        "IfcTransportElement",
+    ),
+    "IfcDistributionSystem": ("IfcDistributionElement",),
+    "IfcStructuralAnalysisModel": ("IfcStructuralMember", "IfcStructuralConnection"),
+    "IfcSystem": ("IfcProduct",),
+    "IfcGroup": ("IfcObjectDefinition",),
+}
+
+
+def is_assignable(product, system) -> bool:
+    for assignable in group_types.get(system.is_a(), ()):
+        if product.is_a(assignable):
+            return True
+    return False
+
+
 def get_system_elements(system):
     results = []
     for rel in system.IsGroupedBy: