This mapper function returns the standard deviation of each sliding window. The location and elevation returned are those associated with the most recent value in the sliding window.
This is similar to
mapper.sd except this version implements Welford's method which is 10% slower but is much less subject to numerical instability.
The standard deviation is the square root of the variance as computed by
mapper.sd.welford function can only be applied to values of type LONG or DOUBLE, when applied to STRING or BOOLEAN it does not return any value.