Add dask array support

[brendancol]

Support for dask arrays will allow for distributed spatial analysis.

Convolution filter dask array pseudo example

def distributed_hillshade(block):
  return hillshade(block)

g = da.overlap.overlap(mydaskarray, depth={0: 2, 1: 2},
                    boundary={0: 'periodic', 1: 'periodic'})
g2 = g.map_blocks(distributed_hillshade)
result = da.overlap.trim_internal(g2, {0: 2, 1: 2})

[brendancol]

Multifile xarray read example:

xr.open_mfdataset('my/files/*.nc', parallel=True)