Use voxel left/right edges, not centres

[bwpearre]

As it stands, the laser is programmed to turn on and off at voxel centres. Correct behaviour might be to turn on at the left edge of an on-voxel and off at the right edge of an on-voxel. Of course that's not quite possible given the frequency of the Pockels control card, and especially given the slower---and nonlinear---response of the Pockels cell. There could be some calibration involved here... possibly via oscilloscope. Non-urgent, since the way I've done it is pretty close to just shifting the whole object a very small amount to the right, but it could lead to some small improvement...