Returns list of variables of the network

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

def get_variables(self):
        """
        Returns list of variables of the network

        Examples
        --------
        >>> reader = XMLBIF.XMLBIFReader("xmlbif_test.xml")
        >>> reader.get_variables()
        ['light-on', 'bowel-problem', 'dog-out', 'hear-bark', 'family-out']
        """
        variables = [variable.find('NAME').text for variable in self.network.findall('VARIABLE')]
        return variables