Returns the edges of the network

        Examples
        --------
        >>> reader = XMLBIF.XMLBIFReader("xmlbif_test.xml")
        >>> reader.get_edges()
        [['family-out', 'light-on'],
         ['family-out', 'dog-out'],
         ['bowel-problem', 'dog-out'],
         ['dog-out', 'hear-bark']]

def get_edges(self):
        """
        Returns the edges of the network

        Examples
        --------
        >>> reader = XMLBIF.XMLBIFReader("xmlbif_test.xml")
        >>> reader.get_edges()
        [['family-out', 'light-on'],
         ['family-out', 'dog-out'],
         ['bowel-problem', 'dog-out'],
         ['dog-out', 'hear-bark']]
        """
        edge_list = [[value, key] for key in self.variable_parents
                     for value in self.variable_parents[key]]
        return edge_list