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.

Why the underscore? Shouldn't the names be simply ``rgb_to_hsv_array_opt`` and ``hsv_to_rgb_array_opt``?