title: Transposing a pandas dataframe with multiple columns to multiple rows
body: <p>I have dataset in the following format</p>
  
  <pre><code>source  adset     date  impression  click   spend
  fb      abc       1/1/19    100 140 20
  fb      abc       1/2/19    200 130 40
  cl      xyz       1/1/19    300 200 50
  cl      xyz       1/2/19    500 200 50
  </code></pre>
  
  <p>how to use pandas to get the format as below?</p>
  
  <pre><code>source  adset        kpi    1/1/19  1/2/19
  fb     abc         impression   100 200
  fb     abc         click    140 130
  fb     abc         spend    20  40
  cl     abc         impression   300 500
  cl     abc         click    200 200
  cl     abc         spend    50  50
  </code></pre>