Added support for NChannelImage and ExtendToNChannel


Sorry for all the previous open/close pull requests. This is the only merge to look at.

Ok. I would need to know what is inside ``jitterfn`` before I can suggest how to control the random state. 

I also need to know what it is you want to commit to Topographica before I can give a suggestion for a file. Is this your improved ``GeneratorSheet``? If so maybe into ``topo/base/generatorsheet.py`` but that also depends on the imports needed (as topo.base is supposed to have minimal dependencies).