getBaseEdge(clusteredEdgeId) {
    // Just kludge this by returning the first base edge id found
    return this.getBaseEdges(clusteredEdgeId)[0];
  }
Get the base edge id of clusterEdgeId. cluster edge (clusteredEdgeId) -> cluster edge B -> cluster edge C -> base edge
@param {vis.Edge.id} clusteredEdgeId
@returns {vis.Edge.id} baseEdgeId

TODO: deprecate in 5.0.0. Method getBaseEdges() is the correct one to use.