Memory gotcha with ConvertToTraceXML in python API

[analog-cbarber]

In the Python API, t appears that ClientXML.ConvertToTraceXML requires you to make sure to keep a reference to the ClientXML object or else you will get a segfault.

    child = ClientXML()
    parent.GetChild(child, 0)
    trace = child.ConvertToTraceXML()
    child = None # can trigger GC
    trace.IsTagWme() # seg

It would be nice if this just didn't blow up like this, but short of that, it would be good to document this.