title: How to DataFrame.groupby along axis=1
body: <p>I have:</p>
  
  <pre><code>df = pd.DataFrame({'A':[1, 2, -3],'B':[1,2,6]})
  df
      A   B
  0   1   1
  1   2   2
  2   -3  6
  </code></pre>
  
  <p>Q: How do I get:</p>
  
  <pre><code>    A
  0   1
  1   2
  2   1.5
  </code></pre>
  
  <p>using <code>groupby()</code> and <code>aggregate()</code>?</p>
  
  <p>Something like,</p>
  
  <pre><code>df.groupby([0,1], axis=1).aggregate('mean')
  </code></pre>
  
  <p>So basically groupby along <code>axis=1</code> and use row indexes <code>0</code> and <code>1</code> for grouping. (without using Transpose)</p>