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.

Ok.

I'll leave the underscore issue to Jim. To me, it is no longer internal because you are using these underscores in a new file. I don't feel particularly strongly about this issue though.

For the formatting of C code, that is a stylistic issue and I am not a C programmer. Given that the C code is in its own file, I'm happy for it to be left the way it is.