Pushes onto the stack an AGGREGATOR which returns the N^th^ percentile of the values in the sliding window.
The returned location and elevation are those of the chosen value or are interpolated in the same way the value is.
The percentile is computed according to the formulae in Hyndman, Rob & Fan, Yanan. (1996). Sample Quantiles in Statistical Packages. The American Statistician. Most used types are 1 when no interpolation is wanted and 7 when interpolation is wanted.