Allow subclassing of some types

Currently, the generated python types are not allowed to be subclassed in Python.

Making python types subclassable involves adding an instance dictionary to them. This implies that tp_traverse needs to be implemented. In addition, the C/C++ object need to become inter-dependent, so that the instance state isn't lost when the python wrapper goes out of scope, but the C/C++ object is kept alive and to-python convertion is needed again at a later stage. This means that python reference counting will no longer be enough to delete the object.

In summary, making a class subclassable has a runtime cost, and therefore should be disabled by default.