Adds Edges to the probnet dict.

        Parameters
        ----------
        edge: <Element Link at Links Node in XML>
            etree Element consisting Variable tag.

        Examples
        -------
        >>> reader = ProbModelXMLReader()
        >>> reader.add_edge(edge)

def add_edge(self, edge):
        """
        Adds Edges to the probnet dict.

        Parameters
        ----------
        edge: <Element Link at Links Node in XML>
            etree Element consisting Variable tag.

        Examples
        -------
        >>> reader = ProbModelXMLReader()
        >>> reader.add_edge(edge)
        """
        var1 = edge.findall('Variable')[0].attrib['name']
        var2 = edge.findall('Variable')[1].attrib['name']
        self.probnet['edges'][(var1, var2)] = {}
        self.probnet['edges'][(var1, var2)]['directed'] = edge.attrib['directed']
        # TODO: check for the case of undirected graphs if we need to add to both elements of the dic for a single edge.
        if edge.find('Comment') is not None:
            self.probnet['edges'][(var1, var2)]['Comment'] = edge.find('Comment').text
        if edge.find('Label') is not None:
            self.probnet['edges'][(var1, var2)]['Label'] = edge.find('Label').text
        if edge.find('AdditionalProperties/Property') is not None:
            self.probnet['edges'][(var1, var2)]['AdditionalProperties'] = {}
            for prop in edge.findall('AdditionalProperties/Property'):
                self.probnet['edges'][(var1, var2)]['AdditionalProperties'][prop.attrib['name']] = prop.attrib['value']