Changes:修正版迭代方差计算，消除了无限递增的问题，但是具体计算公式需要后期再确认

variance_x.py

@@ -27,6 +27,6 @@ class variance_x(algorithm):
             self.var_ = 0
         else:
             present_avg_ = self.avg_ * (self.window_length_ - 1)/self.window_length_ + value / self.window_length_
-            self.var_ = self.var_ +(value - present_avg_) * (value-self.avg_)
+            self.var_ = self.var_ +(value - present_avg_) * (value-self.avg_) / (present_avg_ - self.avg_)
             self.avg_ = present_avg_
         return self.var_