docs: 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