This mapper function multiplies by a constant the single value in a sliding window.
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.mul function can be applied to data of type LONG or DOUBLE.
mapper.mul function must be parameterized with the constant.
The result will be of the same type as the constant.