Add property to variables in BIF

        Returns
        -------
        dict: dict of type {variable: list of properties }

        Example
        -------
        >>> from pgmpy.readwrite import BIFReader, BIFWriter
        >>> model = BIFReader('dog-problem.bif').get_model()
        >>> writer = BIFWriter(model)
        >>> writer.get_properties()
        {'bowel-problem': ['position = (335, 99)'],
         'dog-out': ['position = (300, 195)'],
         'family-out': ['position = (257, 99)'],
         'hear-bark': ['position = (296, 268)'],
         'light-on': ['position = (218, 195)']}

def get_properties(self):
        """
        Add property to variables in BIF

        Returns
        -------
        dict: dict of type {variable: list of properties }

        Example
        -------
        >>> from pgmpy.readwrite import BIFReader, BIFWriter
        >>> model = BIFReader('dog-problem.bif').get_model()
        >>> writer = BIFWriter(model)
        >>> writer.get_properties()
        {'bowel-problem': ['position = (335, 99)'],
         'dog-out': ['position = (300, 195)'],
         'family-out': ['position = (257, 99)'],
         'hear-bark': ['position = (296, 268)'],
         'light-on': ['position = (218, 195)']}
        """
        variables = self.model.nodes()
        property_tag = {}
        for variable in sorted(variables):
            properties = self.model.node[variable]
            properties = collections.OrderedDict(sorted(properties.items()))
            property_tag[variable] = []
            for prop, val in properties.items():
                property_tag[variable].append(str(prop) + " = " + str(val))
        return property_tag