This mapper function raises the single value in a sliding window to a constant power.
If the sliding window is empty, the function does not return any value. If the sliding window contains more than a single value, an error is produced.
The associated location and elevation are those of the single value.
mapper.pow function can be applied to data of type LONG or DOUBLE.
mapper.pow function must be parameterized with the constant.
The result is always of type DOUBLE.