'Transforms the xml_data from <$custom_tag attr="" attr=""/> to
<customtag attr="" attr="" impl="$custom_tag"/>'

@classmethod
def from_xml(cls, xml_data, system, id_generator):

    xml_object = etree.fromstring(xml_data)
    system.error_tracker('WARNING:    the    <{tag}>    tag    is    deprecated.        Instead,    use    <customtag    impl="{tag}"    attr1="..."    attr2="..."/>.    '.format(tag=xml_object.tag))
    tag = xml_object.tag
    xml_object.tag = 'customtag'
    xml_object.attrib['impl'] = tag
    return system.process_xml(etree.tostring(xml_object))
