using a cyclic sequence to model a limited history

trend.py

   def __init__(self) :
        ....
        self.history = Moving_Sequence(n)
        ...
   def monitor...
       for baro in reader:
           ...
           self.history.add(baro)
           ...

   def updateTrend(self) :
        self.trend = self.history.sum(0)
        if self.trend is not None
            self.tendency = conv.trend_to_tendency(self.trend)