title: Filter numpy array according to predicate of position
body: <p>Suppose I have a two dimensional numpy array with a given <code>shape</code> and I would like to get a view of the values that satisfy a predicate based on the value's position. That is, if <code>x</code> and <code>y</code> are the column and row index accordingly and a predicate <code>x&gt;y</code> the function should return only the array's values for which the column index is greater than the row index. </p>
  
  <p>The easy way to do is a double loop but I would like a possibly faster (vectorized maybe?) approach?</p>
  
  <p>Is there a better way? </p>