vastly simplify Dataset.to_dataframe() by using broadcast_variables
vastly simplify Dataset.to_dataframe() by using broadcast_variables

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