title: Why skimage mean filter does not work on float array?
body: <p>I am going to apply a mean filter on an array of float with <code>window_size=3</code> for example. I have found this library:</p>
  
  <pre><code>from skimage.filters.rank import mean
  import numpy as np
  
  x=np.array([[1,8,10],
             [5,2,9],
             [7,2,9],
             [4,7,10],
             [6,14,10]])
  
  print(x)
  print(mean(x, square(3)))
  
  
  [[ 1  8 10]
   [ 5  2  9]
   [ 7  2  9]
   [ 4  7 10]
   [ 6 14 10]]
  [[ 4  5  7]
   [ 4  5  6]
   [ 4  6  6]
   [ 6  7  8]
   [ 7  8 10]]
  </code></pre>
  
  <p>but this function can't run on float arrays:</p>
  
  <pre><code>from skimage.filters.rank import mean
  import numpy as np
  
  x=np.array([[1,8,10],
             [5,2,9],
             [7,2,9],
             [4,7,10],
             [6,14,10]])
  
  print(x)
  print(mean(x.astype(float), square(3)))
  
  File "/home/pd/RSEnv/lib/python3.5/site-packages/skimage/util/dtype.py", line 236, in convert
  raise ValueError("Images of type float must be between -1 and 1.")
      ValueError: Images of type float must be between -1 and 1.
  </code></pre>
  
  <p>How to solve this?</p>