collapse a pandas MultiIndex
<p>Suppose I have a <code>DataFrame</code> with <code>MultiIndex</code> columns.  How can I collapse the levels to a concatenation of the values so that I only have one level?</p>

<h3>Setup</h3>

<pre><code>np.random.seed([3, 14])
col = pd.MultiIndex.from_product([list('ABC'), list('DE'), list('FG')])
df = pd.DataFrame(np.random.rand(4, 12) * 10, columns=col).astype(int)

print df

   A           B           C         
   D     E     D     E     D     E   
   F  G  F  G  F  G  F  G  F  G  F  G
0  2  1  1  7  5  9  9  2  7  4  0  3
1  3  7  1  1  5  3  1  4  3  5  6  0
2  2  6  9  9  9  5  7  0  1  2  7  5
3  2  2  8  0  3  9  4  7  0  8  2  5
</code></pre>

<p>I want the result to look like this:</p>

<pre><code>   ADF  ADG  AEF  AEG  BDF  BDG  BEF  BEG  CDF  CDG  CEF  CEG
0    2    1    1    7    5    9    9    2    7    4    0    3
1    3    7    1    1    5    3    1    4    3    5    6    0
2    2    6    9    9    9    5    7    0    1    2    7    5
3    2    2    8    0    3    9    4    7    0    8    2    5
</code></pre>