Fix #1576. Managing IfcSystem subtypes is now possible.

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

@@ -23,13 +23,13 @@ import ifcopenshell.api
 class Usecase:
     def __init__(self, file, **settings):
         self.file = file
-        self.settings = {}
+        self.settings = {"ifc_class": "IfcSystem"}
         for key, value in settings.items():
             self.settings[key] = value
 
     def execute(self):
         return self.file.create_entity(
-            "IfcSystem",
+            self.settings["ifc_class"],
             **{
                 "GlobalId": ifcopenshell.guid.new(),
                 "OwnerHistory": ifcopenshell.api.run("owner.create_owner_history", self.file),