This class maintains mean and variation for any sequence of input provided to it.
It is initialized with number of rolling periods which basically means the number of past
inputs whose data will be considered to maintain mean and variation.
It will use O(N) memory to maintain these statistics, where N is number of look up periods it
was initialized with.
If zero is passed during initialization then it will maintain mean and variance from the
start. It will use O(1) memory only. But note that since it will maintain mean / variance
from the start the statistics may behave like constants and may ignore short trends.
All operations are O(1) except the initialization which is O(N).