title: Removing columns which has only "nan" values from a NumPy array
body: <p>I have a <code>NumPy</code> matrix like the one below:</p>
  
  <pre><code>[[182 93 107 ..., nan nan -1]
   [182 93 107 ..., nan nan -1]
   [182 93 110 ..., nan nan -1]
   ..., 
   [188 95 112 ..., nan nan -1]
   [188 97 115 ..., nan nan -1]
   [188 95 112 ..., nan nan -1]]
  </code></pre>
  
  <p>I want to remove the columns which only involve <code>nan</code> values from the above matrix.</p>
  
  <p>How can I do this? Thanks.</p>