Sigma Stateful function should have rolling window period

The stateful sigma function is useful in that it computes deviations from the norm easily, but currently it doesn't support limiting the window that it tracks. Meaning it computes the moving mean and stddev for all points seen forever. For it to be more useful, older values need to be forgotten via a moving window. Essentially this should look like the moving_average function but with the sigma pieces added on.

Start by locating the stateful sigma and moving_average function implementations and their tests in the Kapacitor repository. Compare how moving windows are represented and tested, then define done as sigma forgetting values outside the configured rolling window while preserving its moving mean and standard-deviation behavior.

Written by the indexing model from the issue text.