title: How to create separate rows for each of node link in a network data frame
body: <p>I am creating a directed network diagram and want to create a data frame which has one row for each link. How can I build it from a data frame that only has two columns with the source and the destinations? </p>
  
  <p>I have a From column, which is the source and To column which involves lists of each destination.</p>
  
  <pre><code># What I have:
  From            To
  A               [B,C,D]
  B               [C,D]
  C               [A]
  
  # What I need:
  Source          Destination
  A               B
  A               C
  A               D
  B               C
  B               D
  C               A
  </code></pre>