Fix performance regression for decode_cf_variable
Fix performance regression for decode_cf_variable

We were passing the netCDF4 Variable directly to decode_cf_datetime, which
does a very expensive np.asarray() call (fixed in the master branch of
netCDF4-python). By passing the data as a numpy array, this call is *much*
faster.

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