Average.h: Use of circular buffer and sum variable for better performance

(1) A circular buffer should be used to store the array values. This removes the requirement for shifting values in push().
(2) The sum of values should be updated continuously to accelerate the mean() calculation.

--> The functions push() and mean() don't have to loop over the array elements.
Merge pull request #2 from catox0/patch-2

Average.h: Use of circular buffer and sum variable for better performanc...