Support auto detection of unit type when adding imperial units

src/ifcopenshell-python/test/api/unit/test_edit_derived_unit.py

@@ -0,0 +1,12 @@
+import test.bootstrap
+import ifcopenshell.api
+
+
+class TestEditDerivedUnit(test.bootstrap.IFC4):
+    def test_run(self):
+        unit = self.file.createIfcDerivedUnit()
+        ifcopenshell.api.run("unit.edit_derived_unit", self.file, unit=unit, attributes={
+            "UnitType": "USERDEFINED", "UserDefinedType": "UserDefinedType"
+        })
+        assert unit.UnitType == "USERDEFINED"
+        assert unit.UserDefinedType == "UserDefinedType"