Add cube selem.
Add ball selem. I verified in 2D that this usage of np.mgrid yields the exact same results as np.meshgrid (np.meshgrid is not available for 3D).
Fix typo in docstring of disk().
Adding octahedron selem (3D equivalent of diamond).
Added test for ball and octahedton selems.

Note that only the center slice (in each dimension is tested)
since there is no reference data available for these selems.
I did visualize the 3D shapes to see whether they look right.
Merge pull request #484 from almarklein/3d-selem

Adding predefined structuring elements for 3D morphology.