Proper fix for error 'object has no attribute 'getSlave' in EtherCAT extension

traceback:
File "/home/developer/WorkData/PLC/beremiz/beremiz/IDEFrame.py", line 1433, in OnPouSelectedChanged
window.RefreshView()
File "/home/developer/WorkData/PLC/beremiz/beremiz/etherlab/ConfigEditor.py", line 837, in RefreshView
self.RefreshProcessVariables()
File "/home/developer/WorkData/PLC/beremiz/beremiz/etherlab/ConfigEditor.py", line 886, in RefreshProcessVariables
slaves = self.Controler.GetSlaves(**self.CurrentNodesFilter)
File "/home/developer/WorkData/PLC/beremiz/beremiz/etherlab/EthercatMaster.py", line 341, in GetSlaves
for slave in self.Config.getConfig().getSlave():
<type 'exceptions.AttributeError'>:_'lxml.etree._Element'_object_has_no_attribute_'getSlave'

Steps to reproduce problem:

- Add new EtherCAT master
- Add new EthercatNode to the master
- double click on


Revert commit "Dirty fix for error '_object_has_no_attribute_'getSlave' in EtherCAT extension"
[a3ac46366b86a0b237dac93be6b2281ac70b98a8].

The problem was that XML elements (proxy object) in some cases were created using custom XML
classes constructors and lxml.etree.Element() call and live python
patching. This causes that lxml backend doesn't know that custom python class
should be used for these XML elements.
Proxy object can be move/deleted and recreated by lxml
backend at any point in time or this can be done in python by copy/deepcopy operations.
If this happens, then newly created
proxy elements are using default class lxml.etree._Element. And all
custom functionality is lost.

All created XML elements should be always created through corresponding
parser and class lookup callback done by lxml backend.
It's described in more details in lxml documentation:
https://lxml.de/element_classes.html

/* This project contains wamp client config to be loaded at runtime startup. */
./project_files/wampconf.json

wampconf.json is in "Project Files", so it is copied to runtime's working directory, and then loaded after program transfer + runtime restart.

Otherwise, wamp config file path can be forced :
./Beremiz_service.py -c /path/to/my/wampconf.json /working/dir

/* Crossbar install */
#sudo apt-get update
#sudo apt-get -y dist-upgrade
sudo apt-get -y install build-essential libssl-dev libffi-dev libreadline-dev libbz2-dev libsqlite3-dev libncurses5dev
sudo apt-get -y install python3-pip
sudo python3 -m pip install -U pip
sudo pip3 install crossbar
crossbar version

/* Start Crossbar command: */
crossbar start

/* Crossbar test router configuration is available in .crossbar directory. */
Tested on version:

    :::::::::::::::::
          :::::          _____                      __
    :::::   :   :::::   / ___/____ ___   ___  ___  / /  ___ _ ____
    :::::::   :::::::  / /__ / __// _ \ (_-< (_-< / _ \/ _ `// __/
    :::::   :   :::::  \___//_/   \___//___//___//_.__/\_,_//_/
          :::::
    :::::::::::::::::   Crossbar v18.7.2

    Copyright (c) 2013-2018 Crossbar.io Technologies GmbH, licensed under AGPL 3.0.

 Crossbar.io        : 18.7.2
   Autobahn         : 18.7.1
   Twisted          : 18.7.0-EPollReactor
   LMDB             : 0.94/lmdb-0.9.22
   Python           : 3.6.6/CPython
 Frozen executable  : no
 Operating system   : Linux-4.16.0-2-rt-amd64-x86_64-with-debian-buster-sid
 Host machine       : x86_64
 Release key        : RWS9T4NltFjmKSMbEtETnOMxRjLhOEZ6e80T5MYzTTh/+NP9Jk20sJmA