input: Write a python function to convert Fahrenheit to Celsius using a class.
  assert TemperatureConverter().set_temp(32).convert_temp() == 0.0
  assert TemperatureConverter().set_temp(212).convert_temp() == 100.0
  assert TemperatureConverter().set_temp(98.6).convert_temp() == 37.0
output: class TemperatureConverter:
      def __init__(self):
          self.fahrenheit = None
  
      def set_temp(self, fahrenheit):
          self.fahrenheit = fahrenheit
          return self
  
      def convert_temp(self):
          return (self.fahrenheit - 32) * 5 / 9