Added code for new multichannel retina

This PR introduces two new classes related to measurement code for the new multichannel retina.
The hue measurement code is written by Giacomo Spigler. Using the same principles, I adapted this code to also work with ocularity.

As mentioned in https://github.com/ioam/topographica/pull/584, Giacomo mentioned once that he has some code which make these classes obsolete, by checking whether the input is a multichannel object. However, I have not heard from him regarding this issue since then. It would be great, if Giacomo finalizes this code.