Return dask arrays?

[ks905383]

Since impl='numba' now supports dask, it would be great if there was a way to return dask arrays instead of forcing compute before putting aggregated results into wm.agg.

It would probably require a substantial rewrite of wm.agg.

But the idea would be that

with xa.set_options(impl='numba'):
   wm = xa.pixel_overlaps(ds,gdf)
   agg = xa.aggregate(ds,wm)
   ds_out = agg.to_dataset()

returns a dask array.