title: How to search entire Pandas dataframe for a string and get the name of the column that contains it?
body: <p>I want to find <em>the name of the column</em> in a dataframe ("categories") that contains a given string.</p>
  
  <pre><code>categories
  
      Groceries   Electricity Fastfood    Parking 
  0   SHOP        ELCOMPANY   MCDONALDS   park
  1   MARKET      ELECT       Subway      car
  2   market      electr      Restauran   247 
  </code></pre>
  
  <p>Say I want to search this entire dataframe for string "MCDO". <strong>The answer should be "Fastfood"</strong>. I tried using str.contains but it doesn't seem to work for dataframes.</p>
  
  <p>How can I achieve this? Thank you.</p>