title: Creating Multi -Column Index for a Dataframe
body: <p>Is it possible to change a single level column dataframe to a multi-column dataframe? If we have a dataframe like this,</p>
  <pre><code>import pandas as pd
      
  df = pd.DataFrame({
      'a': [0, 1, 2, 3],
      'b': [4, 5, 6, 7],
      'c': [3, 5, 6, 2],
      'd': [1, 5, 7, 0],
  })
  </code></pre>
  <p>can we change it's column names as below?. So, briefly what I am trying to do is to have 2-levels of column index without changing the values of the dataframe.</p>
  <pre><code>    A       B
      a   b   c   d
  0   0   4   3   1
  1   1   5   5   5
  2   2   6   6   7
  3   3   7   2   0
  </code></pre>
  <p>Any help?</p>