_restoreFrozenNodes() {
    const nodes = this.body.nodes;
    for (const id in nodes) {
      if (Object.prototype.hasOwnProperty.call(nodes, id)) {
        if (this.freezeCache[id] !== undefined) {
          nodes[id].options.fixed.x = this.freezeCache[id].x;
          nodes[id].options.fixed.y = this.freezeCache[id].y;
        }
      }
    }
    this.freezeCache = {};
  }
Unfreezes the nodes that have been frozen by _freezeDefinedNodes.
@private