title: creating a edgelist from a dataframe
body: <p>I am trying to get from this:</p>
  
  <pre><code>session location sequence weight INDIVIDUAL action
  a1      texas    1        10       john       Z1
  a1      texas    2        5        peter      Z2
  a1      texas    3        3        ben        Z1
  a1      texas    4        5        peter      Z5
  a2      calif    1        25       esther     Z3
  a2      calif    2        5        peggy      Z2
  a2      calif    3        10       greg       Z5
  </code></pre>
  
  <p>to this:</p>
  
  <pre><code>INDIVIDUAL1 INDIVIDUAL2 weight
  john        peter       10
  john        ben         10
  peter       john        5
  peter       ben         5
  ben         john        3
  ben         peter       3
  peter       john        5
  peter       ben         5        
  </code></pre>
  
  <p>I am exploring a number of options, including the use of for-loops, but I am a little concern that it may take too long as my dataset gets really big. Any pointers greatly appreciated!</p>
  
  <p>Thank you!</p>