Added support for NChannelImage and ExtendToNChannel

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

Shouldn't there be a base class that combines whatever code matches between ExtendToNChannel and NChannelImage?  I don't see any shared parameters right now, but both of them set channel_data=[] in the constructor, both of them should eventually support a list of names for the channels in the same way, and I would think that both of them should apply output functions in the same way. 

If so, then this new base class would be the thing called pattern.NChannel, not what is currently called ExtendToNChannel. 

At that point, ExtendToNChannel could be renamed to NChannelWrapper (since it wraps an existing PatternGenerator as an NChannel object), but ExtendToNChannel also seems ok.  And both this and NChannelImage would then inherit from pattern.NChannel. 





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