This mapper function computes the vertical speed between the first and last readings of the sliding window. It does not compute the speed based on the total
vertical distance traveled in the sliding window, it only considers its extrema (it differs significantly of what is done for horizontal speed in
mapper.hspeed), thus the result can be positive or negative.
If one of the extrema of the sliding window has no elevation, no value is returned.
The associated location and elevation are those at the tick being computed if it is part of the sliding window.
The computed speed is expressed in meters per second.
mapper.vspeed function can be applied to data of any type since it only considers elevations.