title: selecting edges based on source/target in igraph
body: <p>is there an easy way to select/delete edges based on their source and target in igraph?</p>
  
  <p>what I am using is essentially</p>
  
  <pre><code>g.es["source"] = [e.source for e in g.es]
  g.es["target"] = [e.target for e in g.es]    
  g.es["tuple"]  = [e.tuple  for e in g.es]        
  
  g.es.select(target=root)
  </code></pre>
  
  <p>but I feel like there should be a way to do that without storing source/target info twice.</p>