Fix inconsistencies in examples and many more improvements

doc/examples/plot_circular_hough_transform.py

@@ -29,8 +29,6 @@ original picture in order to detect centers outside the frame.
 Its size is extended by two times the larger radius.
 
 """
-
-
 import numpy as np
 import matplotlib.pyplot as plt
 
@@ -39,6 +37,7 @@ from skimage.transform import hough_circle
 from skimage.feature import peak_local_max
 from skimage.draw import circle_perimeter
 
+
 # Load picture and detect edges
 image = data.coins()[0:95, 70:370]
 edges = filter.canny(image, sigma=3, low_threshold=10, high_threshold=50)