Add outcome to variables of XMLBIF

        Return
        ------
        dict: dict of type {variable: outcome tags}

        Examples
        --------
        >>> writer = XMLBIFWriter(model)
        >>> writer.get_states()
        {'dog-out': [<Element OUTCOME at 0x7ffbabfcdec8>, <Element OUTCOME at 0x7ffbabfcdf08>],
         'family-out': [<Element OUTCOME at 0x7ffbabfd4108>, <Element OUTCOME at 0x7ffbabfd4148>],
         'bowel-problem': [<Element OUTCOME at 0x7ffbabfd4088>, <Element OUTCOME at 0x7ffbabfd40c8>],
         'hear-bark': [<Element OUTCOME at 0x7ffbabfcdf48>, <Element OUTCOME at 0x7ffbabfcdf88>],
         'light-on': [<Element OUTCOME at 0x7ffbabfcdfc8>, <Element OUTCOME at 0x7ffbabfd4048>]}

def get_states(self):
        """
        Add outcome to variables of XMLBIF

        Return
        ------
        dict: dict of type {variable: outcome tags}

        Examples
        --------
        >>> writer = XMLBIFWriter(model)
        >>> writer.get_states()
        {'dog-out': [<Element OUTCOME at 0x7ffbabfcdec8>, <Element OUTCOME at 0x7ffbabfcdf08>],
         'family-out': [<Element OUTCOME at 0x7ffbabfd4108>, <Element OUTCOME at 0x7ffbabfd4148>],
         'bowel-problem': [<Element OUTCOME at 0x7ffbabfd4088>, <Element OUTCOME at 0x7ffbabfd40c8>],
         'hear-bark': [<Element OUTCOME at 0x7ffbabfcdf48>, <Element OUTCOME at 0x7ffbabfcdf88>],
         'light-on': [<Element OUTCOME at 0x7ffbabfcdfc8>, <Element OUTCOME at 0x7ffbabfd4048>]}
        """
        outcome_tag = {}
        cpds = self.model.get_cpds()
        for cpd in cpds:
            var = cpd.variable
            outcome_tag[var] = []
            if cpd.state_names is None or cpd.state_names.get(var) is None:
                states = range(cpd.get_cardinality([var])[var])
            else:
                states = cpd.state_names[var]

            for state in states:
                state_tag = etree.SubElement(self.variables[var], "OUTCOME")
                state_tag.text = self._make_valid_state_name(state)
                outcome_tag[var].append(state_tag)
        return outcome_tag