title: Reshape dataframe by reversing columns names with columns values
body: <p>I am trying to reshape a df so that the values become columns and the current columns' names become the values of the new df.</p>
  <p>Here is an exemple.</p>
  <pre><code>df1=pd.DataFrame(data=[['v1','v2','v3'],['v4','v3','v1'],[np.nan,'v2','v1'],['v5','v3','v6'],], columns=['A','B','C'], index=['d1','d2','d3','d4'])
  df1.index.names=['Day']
  df1 # What I have
  Out[1]: 
         A   B   C
  Day             
  d1    v1  v2  v3
  d2    v4  v3  v1
  d3   NaN  v2  v1
  d4    v5  v3  v6
  </code></pre>
  <pre><code>df2=pd.DataFrame(data=[['A','B','C',np.nan,np.nan,np.nan],['C',np.nan,'B','A',np.nan,np.nan],['C','B',np.nan,np.nan,np.nan,np.nan],[np.nan,np.nan,'B',np.nan,'A','C']], columns=['v1','v2','v3','v4','v5','v6'], index=['d1','d2','d3','d4'])
  df2.index.names=['Day']
  df2 # Desired output
  Out[2]: 
       v1   v2   v3   v4   v5   v6
  d1    A    B    C  NaN  NaN  NaN
  d2    C  NaN    B    A  NaN  NaN
  d3    C    B  NaN  NaN  NaN  NaN
  d4  NaN  NaN    B  NaN    A    C
  </code></pre>
  <p>I guess something with stack(), unstack() or pivot()?</p>