Added support for NChannelGeneratorSheet and optimized color conversion


Optimized colorconversion added to topo.optimized.color (new file) and topo.analysis.__init (overriding featuremapper.colorspaces default unoptimized functions).

NChannelGeneratorSheet added to topo.base.generatorsheet.

I agree that it would be great to separate this code from the rest of the class, but unfortunately it applies across all of the channels, whereas output_fns work only on one channel at a time.  It does need a new name, though, as there is no retina in imagen.