Fix and add output for timing calls

doc/source/user_guide/numpy_images.txt

@@ -172,10 +172,13 @@ 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(t1 - t0, "seconds")
+    >>> print("%.2f seconds" % (t1 - t0))
+    0.14 seconds
     >>> im3d_t = np.transpose(im3d).copy()
     >>> im3d_t.shape
     (1024, 1024, 100)
     >>> s0 = time.time(); x = out_of_order_multiply(im3d, 5); s1 = time.time()
-    >>> print(s1 - s0, "seconds")
-    >>> print("Speedup: ", (s1 - s0) / (t1 - t0))
+    >>> print("%.2f seconds" % (s1 - s0))
+    1.18 seconds
+    >>> print("Speedup: %.1fx" % ((s1 - s0) / (t1 - t0)))
+    Speedup: 8.6x