Returns the states of variables present in the network

        Examples
        --------
        >>> reader = XMLBIF.XMLBIFReader("xmlbif_test.xml")
        >>> reader.get_states()
        {'bowel-problem': ['true', 'false'],
         'dog-out': ['true', 'false'],
         'family-out': ['true', 'false'],
         'hear-bark': ['true', 'false'],
         'light-on': ['true', 'false']}

def get_states(self):
        """
        Returns the states of variables present in the network

        Examples
        --------
        >>> reader = XMLBIF.XMLBIFReader("xmlbif_test.xml")
        >>> reader.get_states()
        {'bowel-problem': ['true', 'false'],
         'dog-out': ['true', 'false'],
         'family-out': ['true', 'false'],
         'hear-bark': ['true', 'false'],
         'light-on': ['true', 'false']}
        """
        variable_states = {variable.find('NAME').text: [outcome.text for outcome in variable.findall('OUTCOME')]
                           for variable in self.network.findall('VARIABLE')}
        return variable_states