clarify the type of downscaling that is currently implemented #1

At the moment only reductions over contiguous non-overlapping regions are supported, e.g. windowed averaging. From a signal processing perspective this is not the ideal method for downscaling -- it would be better to apply a gaussian filter to attenuate out-of-band frequencies and then downsample. However this is not possible as long as dask.array.coarsen is used to generate the downscaled array. Look into using dask.array.map_overlap or the gaussian smoothing functionality in dask-image to implement a gaussian pyramid.

cc @mkitti