Add states to variable of BIF

        Returns
        -------
        dict: dict of type {variable: a list of states}

        Example
        -------
        >>> from pgmpy.readwrite import BIFReader, BIFWriter
        >>> model = BIFReader('dog-problem.bif').get_model()
        >>> writer = BIFWriter(model)
        >>> writer.get_states()
        {'bowel-problem': ['bowel-problem_0', 'bowel-problem_1'],
         'dog-out': ['dog-out_0', 'dog-out_1'],
         'family-out': ['family-out_0', 'family-out_1'],
         'hear-bark': ['hear-bark_0', 'hear-bark_1'],
         'light-on': ['light-on_0', 'light-on_1']}

def get_states(self):
        """
        Add states to variable of BIF

        Returns
        -------
        dict: dict of type {variable: a list of states}

        Example
        -------
        >>> from pgmpy.readwrite import BIFReader, BIFWriter
        >>> model = BIFReader('dog-problem.bif').get_model()
        >>> writer = BIFWriter(model)
        >>> writer.get_states()
        {'bowel-problem': ['bowel-problem_0', 'bowel-problem_1'],
         'dog-out': ['dog-out_0', 'dog-out_1'],
         'family-out': ['family-out_0', 'family-out_1'],
         'hear-bark': ['hear-bark_0', 'hear-bark_1'],
         'light-on': ['light-on_0', 'light-on_1']}
        """
        variable_states = {}
        cpds = self.model.get_cpds()
        for cpd in cpds:
            variable = cpd.variable
            variable_states[variable] = []
            for state in range(cpd.get_cardinality([variable])[variable]):
                variable_states[variable].append(str(variable) + '_' + str(state))
        return variable_states