Added an example of rolling_window with weights.

Added an example of using the new weights keyword argument to `iris.cube.Cube.rolling_window` to apply a digital filter.

This builds in the gallery on my machine. However, `make extest` reports multiple failures on my machine, so if some one could run the tests elsewhere that would probably be a good idea.

I have made a PR to the iris-sample-data repo to add the data file required by this example.