This mapper function computes the rate of change between the first and last values (rate = (last - first) / (lastick - firsttick)) of each sliding window. The location and elevation returned are those associated with the most recent value in the sliding window.
mapper.rate function can only be applied to values of type LONG or DOUBLE, when applied to STRING or BOOLEAN it does not return any value. If the
sliding window contains less than 2 values, the rate of change will be 0.0.
The rate of change is expressed in units per second.