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.
Not sure.  param.List doesn't inherit from Dynamic, but I guess in principle it could.  Even if it did, though, it would be the List that would be dynamic, not the items on it.  I suppose we could have a new type ListOfDynamic, but seems like the current approach is better than trying to explain and document that.  So I'd just remove the comment, and instead say that this is needed because a List does not support dynamic values directly.