Add doctest skip directives to timing calls

doc/source/user_guide/numpy_images.txt

@@ -151,7 +151,8 @@ the third imaging dimension has lower resolution than the other two,
 and processing must be done frame-wise. When frames are the leading
 dimension, we can use the following syntax:
 
-    >>> for image in image3d: # iterate over first dimension
+    >>> for image in image3d:  # doctest: +SKIP
+    ...     # iterate over the leading dimension (frames)
     ...     do_something_to(image)
 
 
@@ -177,15 +178,15 @@ in a different order, even if the number of operations is the same:
     >>> import time
     >>> im3d = np.random.rand(100, 1024, 1024)
     >>> t0 = time.time(); x = in_order_multiply(im3d, 5); t1 = time.time()
-    >>> print("%.2f seconds" % (t1 - t0))
+    >>> print("%.2f seconds" % (t1 - t0))  # doctest: +SKIP
     0.14 seconds
     >>> im3d_t = np.transpose(im3d).copy() # place "frames" dimension at end
     >>> im3d_t.shape
     (1024, 1024, 100)
     >>> s0 = time.time(); x = out_of_order_multiply(im3d, 5); s1 = time.time()
-    >>> print("%.2f seconds" % (s1 - s0))
+    >>> print("%.2f seconds" % (s1 - s0)) doctest: +SKIP
     1.18 seconds
-    >>> print("Speedup: %.1fx" % ((s1 - s0) / (t1 - t0)))
+    >>> print("Speedup: %.1fx" % ((s1 - s0) / (t1 - t0)))  doctest: +SKIP
     Speedup: 8.6x