Added support for NChannelImage and ExtendToNChannel

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

Isn't it already unnecessary anyway?  I.e., isn't this dating from a time long ago when accessing a numbergenerator caused it to advance, which hasn't been true by default for Topographica for years now?  Even without Jean-Luc's more recent time-dependent stuff, I think it was already safe to read the parameters, though you should check to be sure!