Clean up _setup phase magic in python

[julesghub]

The _setup phase is not run in a obvious parent-child hierarchy, this is a result of the existing AssignFromXML() functions in c.

To simplify this python class confusing we could cull the AssignFromXML() in the c layer - restoring balance to the force. This would improve python readability and reduce the c codebase

[julesghub]

closing because this would be a significant change to the code base. Perhaps for future.