Added support for NChannelImage and ExtendToNChannel


Sorry for all the previous open/close pull requests. This is the only merge to look at.

I don't follow this -- the message seems to say that output_fns are not supported, yet it's raising an exception when there are no output_fns.  In any case, I don't see why output_fns are not supported; surely there should at least be the ability to apply the same output_fn to each channel.