title: python-igraph pickling efficiency
body: <p>I am a beginner in igraph.</p>
  
  <p>I have a graph data of 60000 nodes and 900K edges. I could successfully create the graph using python-igraph and write to disk. My machine has 3G memory. </p>
  
  <p>When I wrote the graph to disk in graphml format, the memory usage was around 19%; with write_pickle, the usage went up to 50% and took significantly more time. </p>
  
  <p>What is the reason behind this behavior of igraph? When should and when should I not use the pickle format? </p>
  
  <p>Please shed light into this.</p>