Y wraparound bug

In ``verticalEDGEs()``, when it iterates over ``y = height - 1`` plane to add connections to ``y = 0`` plane, the step used to switch to the next line is incorrect. It should be exactly ``volume_width * volume_height - volume_width`` (``== next_voxel``), this way, for example we switch from (z, y, x) ``(0, height - 1, width)`` (that's what we have at the end of ``for (i = 0; i < volume_width; i++)`` loop) to ``(1, height - 1, 0)``.