Network: Relax clustering condition for adding already clustered nodes to cluster (#3554)

Previous condition was too strict: if *any* node for the cluster was already clustered, the clustering would abort.

Current fix scans for already clustered nodes and proceeds with what is left.

lib/network/modules/Clustering.js

@@ -524,21 +524,26 @@ class ClusterEngine {
   * @private
   */
   _cluster(childNodesObj, childEdgesObj, options, refreshData = true) {
-    // kill condition: no nodes don't bother
-    if (Object.keys(childNodesObj).length == 0) {return;}
-
-    // allow clusters of 1 if options allow
-    if (Object.keys(childNodesObj).length == 1 && options.clusterNodeProperties.allowSingleNodeCluster != true) {return;}
-
-    // check if this cluster call is not trying to cluster anything that is in another cluster.
+    // Remove nodes which are already clustered
+    var tmpNodesToRemove = []
     for (let nodeId in childNodesObj) {
       if (childNodesObj.hasOwnProperty(nodeId)) {
         if (this.clusteredNodes[nodeId] !== undefined) {
-          return;
+          tmpNodesToRemove.push(nodeId);
         }
       }
     }
 
+    for (var n = 0; n < tmpNodesToRemove.length; ++n) {
+      delete childNodesObj[tmpNodesToRemove[n]];
+    }
+
+    // kill condition: no nodes don't bother
+    if (Object.keys(childNodesObj).length == 0) {return;}
+
+    // allow clusters of 1 if options allow
+    if (Object.keys(childNodesObj).length == 1 && options.clusterNodeProperties.allowSingleNodeCluster != true) {return;}
+
     let clusterNodeProperties = util.deepExtend({},options.clusterNodeProperties);
 
     // construct the clusterNodeProperties