RingResonator Circumference missing vertical racetrack contributions

[rkberlin]

The current implementation of the RingResonator circumference

@property def circumference(self): return 2 * np.pi * self.radius + 2 * self.race_length

lacks the contribution of the optional vertical racetrack section and should be

@property def circumference(self): return 2 * np.pi * self.radius + 2 * self.race_length + 2 * self.vertical_race_length