'Open and parse an xml file.
TODO: Write custom parser to just read the nodes that we need instead of
reading the whole file.
@return xml_tree An xml tree instance. None if error.'

def parse_xml(self, xml_output):

    try:
        parser = ET_ORIG.XMLParser(target=ParserEtToAscii())
        parser.feed(xml_output)
        tree = parser.close()
    except SyntaxError as err:
        print ('SyntaxError:    %s.    %s' % (err, xml_output))
        return None
    return tree
