Fix MAGN-3297 Need Element rebinding in Python nodes

<h6>Summary</h6>

The element binding is right now implemented through the trace mechanism. I have checked that the marshaling of the output of the python functions are correct. But the binding is somehow disabled. I am re-enabling the element binding for the Python nodes so that element binding works correctly in Python nodes.

A test case has been added to check that element binding in Python nodes works.

@pboyer 
PTAL
This code won't reproduce the erroneously rebinding logic.