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.

Add a specific link to the class that it's equivalent to, and saying precisely in what way it is equivalent.  I.e., not "supposed", but in what way it matches, and in what other ways it might not.