title: Flatten DataFrame into a single row
body: <p>I want to reorganize the following mutli-row DataFrame,</p>
  
  <pre><code>       1          2       3
  A  Apple     Orange   Grape
  B    Car      Truck   Plane
  C  House  Apartment  Garage
  </code></pre>
  
  <p>into this, single-row DataFrame.</p>
  
  <pre><code>     1_A     2_A    3_A  1_B    2_B    3_B    1_C        2_C     3_C
  0  Apple  Orange  Grape  Car  Truck  Plane  House  Apartment  Garage
  </code></pre>
  
  <p>Thank you for your help!</p>