Add `val` to calculate updated smoothed value.

def add_value(self, val:float)->None:
        "Add `val` to calculate updated smoothed value."
        self.n += 1
        self.mov_avg = self.beta * self.mov_avg + (1 - self.beta) * val
        self.smooth = self.mov_avg / (1 - self.beta ** self.n)