Sets AdditionalProperties of the ProbModelXML.

def _add_additional_properties(position, properties_dict):
        """
        Sets AdditionalProperties of the ProbModelXML.
        """
        add_prop = etree.SubElement(position, 'AdditionalProperties')
        for key, value in properties_dict.items():
            etree.SubElement(add_prop, 'Property', attrib={'name': key, 'value': value})