Updates for sum_channels
Updates for sum_channels

Make the default sum, with all channels, faster by not performing
the indexing (roughly twice as fast).

Make adding the channels reshape free.

mozata/menpo,mozata/menpo,mozata/menpo,menpo/menpo,patricksnape/menpo,yuxiang-zhou/menpo,yuxiang-zhou/menpo,menpo/menpo,grigorisg9gr/menpo,patricksnape/menpo,yuxiang-zhou/menpo,grigorisg9gr/menpo,mozata/menpo,menpo/menpo,grigorisg9gr/menpo,patricksnape/menpo