Added support for colorspace conversion.

Implementation of the fixes discusses in yesterday's meeting.

I think this is quite complete. I've added support for the more generic "from"-"to" convert in ColorSpace, removed any dependency from Topographica, and added documentation (and the __all__specifier).



Hmm; how will this work in general if it doesn't support sRGB or even RGB?  I.e., don't we regularly use databases that are not in XYZ or LMS format?