smoothing should be 2d rather than 3d

[poldrack]

Currently spatial smoothing in imgmath.py is performed in 3d. however, given the timing offsets between slices, it would be better to do this in 2d. currently this won't have much of an effect, given that the default FWHM of 2.0 is much smaller than the slice thickness, but would be good to implement in principle.