Harmonize all ndimage usage across the library

Only two forms remain in use:

- `from scipy import ndimage as ndi`
- `from scipy.ndimage import function`

doc/examples/plot_peak_local_max.py

@@ -10,7 +10,7 @@ size of the dilation. Locations where the original image is equal to the
 dilated image are returned as local maxima.
 
 """
-from scipy import ndimage
+from scipy import ndimage as ndi
 import matplotlib.pyplot as plt
 from skimage.feature import peak_local_max
 from skimage import data, img_as_float
@@ -19,7 +19,7 @@ im = img_as_float(data.coins())
 
 # image_max is the dilation of im with a 20*20 structuring element
 # It is used within peak_local_max function
-image_max = ndimage.maximum_filter(im, size=20, mode='constant')
+image_max = ndi.maximum_filter(im, size=20, mode='constant')
 
 # Comparison between image_max and im to find the coordinates of local maxima
 coordinates = peak_local_max(im, min_distance=20)