Pushes onto the stack an AGGREGATOR which computes the median, as defined by the NIST, of the values in the sliding window.
It operates on LONG and DOUBLE and it always returns a DOUBLE.
As the median is effectively computed using one or two points, the location/elevation of the result is:
- not defined is there is no location/elevation
- the value of the location/elevation if only one point has this info
- the centroid/mean of the location/elevation if the two points have this info.