Extend Sweeper for multichannel retina + working motion for natural images

The major change in this PR is the extension of Sweeper to work with the new multichannel retina.

The smaller change is that Gaussian patterns in a composite now all have the same orientation, and are therefore swept in the same direction. This change is required to get meaningful direction maps with two direction patches of opposite direction preference within one orientation patch.
That's a good point. Seems like this doesn't happen too often, otherwise I would have suggested to change param accordingly (not really knowing what needs to be changed there though). Seems like this if statement can't be avoided here.