Speedup Variable.concat
Speedup Variable.concat

On this toy benchmark, these changes speed up execution from 240ms to 153ms:

    xs = [xray.Variable('x', [i]) for i in range(10000)]
    %timeit xray.Variable.concat(xs, 'x')

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