Add dask test for to_dataset() and to_array()
Add dask test for to_dataset() and to_array()

cpaulik/xray,kjordahl/xray,chunweiyuan/xarray,hetland/xray,jhamman/xarray,jhamman/xarray,NicWayand/xray,rabernat/xray,clarkfitzg/xray,jhamman/xray,shoyer/xray,markelg/xray,xray/xray,markelg/xray,kjordahl/xray,jcmgray/xarray,drewokane/xray,pydata/xarray,pydata/xarray,shoyer/xarray,hetland/xray,jhamman/xarray,shoyer/xarray,petercable/xray,cpaulik/xray,rabernat/xray,markelg/xray,pydata/xarray,clarkfitzg/xray