title: How to add mean value in pandas between each row
body: <p>Assuming you have a conventional pandas dataframe</p>
  <pre><code>df2 = pd.DataFrame(np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]]),
                     columns=['a', 'b', 'c'])
  </code></pre>
  <p>There I would like to calculate the mean between each row.
  The above pandas dataframe looks like following:</p>
  <pre><code>&gt;&gt;&gt;df2
     a  b  c
  0  1  2  3
  1  4  5  6
  2  7  8  9
  </code></pre>
  <p>Is there an elegant solution to now calculate the mean between each two lines in order to get the following output?</p>
  <pre><code>&gt;&gt;&gt;df2_mean
     a    b    c
  0  1    2    3
  1  2.5  3.5  4.5
  2  4    5    6
  3  5.5  6.5  7.5
  4  7    8    9
  </code></pre>