Fix inconsistencies in examples and many more improvements

doc/examples/plot_ssim.py

@@ -1,4 +1,4 @@
-'''
+"""
 ===========================
 Structural similarity index
 ===========================
@@ -20,12 +20,13 @@ but with very different mean structural similarity indices.
        Transactions on Image Processing, vol. 13, no. 4, pp. 600-612,
        Apr. 2004.
 
-'''
+"""
+import numpy as np
+import matplotlib.pyplot as plt
 
 from skimage import data, color, io, exposure, img_as_float
 from skimage.measure import structural_similarity as ssim
 
-import numpy as np
 
 img = img_as_float(data.camera())
 rows, cols = img.shape
@@ -39,7 +40,6 @@ def mse(x, y):
 img_noise = img + noise
 img_const = img + abs(noise)
 
-import matplotlib.pyplot as plt
 f, (ax0, ax1, ax2) = plt.subplots(1, 3)
 
 mse_none = mse(img, img)