title: Extracting values from last dimension of 3D numpy array
body: <p>I'm trying to extract from a 3D matrix of values a 2d matrix were the last dimension has values from the last dimension of the 3d matrix. For example if 
  P of dimensions [2,2,3] = </p>
  
  <p><code>[
    [[5, 1, 5], [9, 9, 4]], 
    [[0, 9, 8], [8, 6, 8]]
  ]</code> </p>
  
  <p>what is the index matrix in order to get the out matrix
  <code>[[1, 9],[0, 8]]</code> where 1 is the 2nd element of the first column of the first row, 9 is the 1st element of the first row second column, 0 is the 1st element of the second row first column and 8 is the 3rd element of the second row second column?
  The idea is that, for each column I have k different scores. I want to retrieve for each column a different score for which I know the index. </p>
  
  <p>I'm a bit confused with advanced indexing in Numpy and I'm not figuring it out by my self. Thanks!</p>