Resolve issue almende/vis#1857

Perturbing the nodes was being performed on all nodes regardless of whether they were set as fixed.

lib/network/modules/LayoutEngine.js

@@ -234,8 +234,11 @@ class LayoutEngine {
         // perturb the nodes a little bit to force the physics to kick in
         let offset = 70;
         for (let i = 0; i < this.body.nodeIndices.length; i++) {
-          this.body.nodes[this.body.nodeIndices[i]].x += (0.5 - this.seededRandom())*offset;
-          this.body.nodes[this.body.nodeIndices[i]].y += (0.5 - this.seededRandom())*offset;
+          // Only perturb the nodes that aren't fixed
+          if (this.body.nodes[this.body.nodeIndices[i]].predefinedPosition === false) {
+            this.body.nodes[this.body.nodeIndices[i]].x += (0.5 - this.seededRandom())*offset;
+            this.body.nodes[this.body.nodeIndices[i]].y += (0.5 - this.seededRandom())*offset;
+          }
         }
 
         // uncluster all clusters