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

...paces) + NChannelGeneratorSheet
Again, we don't need to put lots of stuff mentioning NChannel or comparing to anything else -- just say what this class does, which makes sense on its own and doesn't need to worry about what any other class does.  I.e., just say it works with the channels() of the PatternGenerator.  The parent class can mention that it uses the ``__call__`` interface of the PatternGenerator, and that it is limited to a single array that will be sent to the Activity port.