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.
Please note that this line results in failing of the Topographica training tests.