Add vision specific featurecoordinators

FeatureCoordinators for disparity, ocularity, spatial frequency and motion are added. This pull request depends on the related ImaGen pull request (https://github.com/ioam/imagen/pull/30). The motion feature coordinator needs time_dependent to be set to true.
There was a reason why I added this code here. We have to get rid of the other legacy code anyway, so I suggest we deal with that in https://github.com/ioam/topographica/issues/562