Adds an edge to the ProbModelXML.

def _add_link(self, edge):
        """
        Adds an edge to the ProbModelXML.
        """
        edge_data = self.data['probnet']['edges'][edge]
        if isinstance(edge, six.string_types):
            edge = eval(edge)
        link = etree.SubElement(self.links, 'Link', attrib={'var1': edge[0], 'var2': edge[1],
                                                            'directed': edge_data['directed']})
        try:
            etree.SubElement(link, 'Comment').text = edge_data['Comment']
        except KeyError:
            pass
        try:
            etree.SubElement(link, 'Label').text = edge_data['Label']
        except KeyError:
            pass
        try:
            self._add_additional_properties(link, edge_data['AdditionalProperties'])
        except KeyError:
            etree.SubElement(link, 'AdditionalProperties')