Pushes onto the stack an AGGREGATOR which returns the N^th^ percentile of the values for each tick.
For this reducer, GTS must be aligned and must have a value for each tick of each GTS. If this reducer encouters a NULL value, ie ticks do not match between Geo Times Series in the same equivalence class, it throws an error.
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.