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.
Do you think it would be ok to simply add these to imagen's feature_coordinators dictionary directly, within topo.misc.featurecoordinators? I.e., just add them to the list of available ones whenever the file gets imported, rather than explicitly listing them here?

If that seems too confusing because of side effects, then it would still be better to have this dictionary defined in topo.misc.featurecoordinators, and then here simply do::

import topo.misc.featurecoordinators.feature_coordinators
imagen.patterncoordinator.PatternCoordinator.feature_coordinators.update(topo.misc.featurecoordinators.feature_coordinators)

Otherwise people will have to edit this file before any new PatternCoordinator can be used, which will be hard for them to realize.