Add fallback decorator for 3D images

We don't support images greater than 2D, so fall back on ndimage
There is a subtle distinction in ndimage between providing a `structure` argument versus a `footprint` argument. I'm not clear on the difference, but I think we want `footprint` in this case.

Still, the results of `skimage` and `scipy.ndimage` are not equivalent:

    AssertionError:
    Arrays are not equal
    (mismatch 25.9259259259%)
     x: array([[  0,   0,   0,   0,   0,   0,   0,   0,   0],
           [  0,   0,   0,   0,   0,   0,   0,   0,   0],
           [  0,   0,   0,  64,  64,  64,   0,   0,   0],...
     y: array([[    0,     0,     0,     0,     0,     0,     0,     0,     0],
           [    0,     0,     0,     0,     0,     0,     0,     0,     0],
           [    0,     0,     0, 16384, 16384, 16384,     0,     0,     0],...