title: Numpy 3d (voxels) erosion
body: <p>I have 3d boolean (numpy) array where element is True if a voxel is present. On an object in this 3d array I want to perform erosion ('eat away the edges', 2d example for <a href="https://docs.opencv.org/2.4/doc/tutorials/imgproc/erosion_dilatation/erosion_dilatation.html" rel="nofollow noreferrer">OpenCV here</a>). I know I can do this by iterating over all elements, but this is very slow. </p>
  
  <p>Is there in Numpy/Scipy a way more native/faster way of achieving this?</p>