Unexpected behaviour in peak_local_max

[Guillaume Gay]

Hi all,

When calling feature.peak_local_max() on a flat image, all the points of 
the image are returned:

 >>> test_image = np.ones((10,10))
 >>> coords = feature.peak_local_max()
 >>> len(coords)
  100

I wouldn't expect that... It originates from two things in the code:

1. The calculated thershold can be higher than the min  of the image:

     corner_threshold = np.max(image.ravel()) * threshold

2. All points higher *or equal* to the threshold are retrieved:

     image_t = (image >= corner_threshold) * 1

So for me this is a bit strange, and is really bad when you have an 
empty frame on a sequence, which tends to happen. Also I think the 
threshold argument is a bit unusual, I more often expect an absolute 
value for the threshold, not a relative one.






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