title: vis.js fixed length edges
body: <p>Is it possible to give an edge a fixed length? Even if I set the length of individual edges, physic engine changes it.</p>
  
  <p>I am trying to visualize 3 clusters, each with couple hundred of nodes. There is an option to aggregate the cluster into couple of nodes. I want to connect these aggregated nodes with really short edges and give these nodes high mass so they will repulse other clusters like they were doing when they had hundreds of nodes.</p>