feature request: mirrored factors

[jeremymanning]

We could support the automatic mirrored factors by adding an "M" factor of the same shape as W, but flipped along the "L" axis. In other words:

• double the number of rows (K) in H to form H_expanded
• factorize X into the convolution of cat(W, M) and H_expanded, where cat(W, M) stacks W and W next to each other along the "K" axis

This would allow us to search for "backwards replay" events that mimic forward replay events, but that are exactly reversed.