Added support for optimized color conversion (extension to imagen.colors...

...paces) + NChannelGeneratorSheet
Sure, we can include text that compares GeneratorSheet and ChannelsGeneratorSheet, saying when you might want to use one, and when you might want to use the other.  But that should be separate from the description of what ChannelsGeneratorSheet does.  I.e., here we should be saying that we take any PatternGenerator, that we use the channels() method to obtain any channels of data provided by it, and that we make those available on various output ports.  We can say this clearly, simply, and without any reference to what other classes do.  Then we can add helpful information saying that people who know they only want a single output should not use this class, and should use GeneratorSheet instead.