_isClusteredEdge(edgeId) {
    return this.clusteredEdges[edgeId] !== undefined;
  }
Determine if edge with given id is not visible due to clustering.

An edge is considered clustered if:
- it is directly replaced by a clustering edge
- any of its connecting nodes is in a cluster
@param {vis.Edge.id} edgeId
@returns {boolean} true if part of a cluster.