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.

skimage/morphology/tests/test_selem.py

@@ -36,9 +36,32 @@ class TestSElem():
                 expected_mask = expected_mask[:, np.newaxis]
             assert_equal(expected_mask, actual_mask)
             k = k + 1
-
+    
+    def strel_worker_3d(self, fn, func):
+        matlab_masks = np.load(os.path.join(data_dir, fn))
+        k = 0
+        for arrname in sorted(matlab_masks):
+            expected_mask = matlab_masks[arrname]
+            actual_mask = func(k)
+            if (expected_mask.shape == (1,)):
+                expected_mask = expected_mask[:, np.newaxis]
+            # Test center slice for each dimension. This gives a good
+            # indication of validity without the need for a 3D reference
+            # mask.
+            c = int(expected_mask.shape[0]/2) 
+            assert_equal(expected_mask, actual_mask[c,:,:])
+            assert_equal(expected_mask, actual_mask[:,c,:])
+            assert_equal(expected_mask, actual_mask[:,:,c])
+            k = k + 1
+    
     def test_selem_disk(self):
         self.strel_worker("disk-matlab-output.npz", selem.disk)
 
     def test_selem_diamond(self):
         self.strel_worker("diamond-matlab-output.npz", selem.diamond)
+    
+    def test_selem_ball(self):
+        self.strel_worker_3d("disk-matlab-output.npz", selem.ball)
+
+    def test_selem_octahedron(self):
+        self.strel_worker_3d("diamond-matlab-output.npz", selem.octahedron)