This mapper function computes the horizontal speed along the path traveled in the sliding window.
The distance is computed by summing the distances along the rhumb line between two consecutive locations.
If there is only one tick in the sliding window, the speed is 0.
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.hspeed function can be applied to data of any type since it only considers locations.