Adding predefined structuring elements for 3D morphology.

Adding cube and ball selems. I could not use meshgrid for the ball, so I used np.mgrid. I verified that the 2D version yields the same result as disk (also for non-integer radia). I tested the 3D version using volume rendering and printing smaller selems to the console.

It would be nice to also create a diamond equivalent in 3D, but I'm not sure what to call it. diamond3 is my best guess...