title: Find cluster/subgraph of nodes in dot
body: <p>I have a dot file that contains a graph having different clusters, something like:</p>
  
  <pre><code>subgraph cluster_bb_5020 {
              style=filled fillcolor=white
              label="label  [5020]"
              NODE230 [label="668"]
              ....
  </code></pre>
  
  <p>I wonder how I can get the cluster number of nodes (e.g. here cluster of  NODE230 is cluster_bb_5020) in the graph by networkx or pydot or pygraphviz? Or in general, how I can get cluster informations?</p>
  
  <p>Thank you!</p>