title: Merging data frame columns of strings into one single column in Pandas
body: <p>I have columns in a dataframe (imported from a CSV) containing text like this.</p>
  
  <pre><code>"New york", "Atlanta", "Mumbai"
  "Beijing", "Paris", "Budapest"
  "Brussels", "Oslo", "Singapore"
  </code></pre>
  
  <p>I want to collapse/merge all the columns into one single column, like this</p>
  
  <pre><code>New york Atlanta
  Beijing Paris Budapest
  Brussels Oslo Singapore
  </code></pre>
  
  <p>How to do it in pandas?</p>