public void attachLinkedChild(AssetManager manager, ModelKey key) {
        addLinkedChild(key);
        Spatial child = manager.loadAsset(key);
        assetChildren.put(key, child);
        attachChild(child);
    }
Add a "linked" child. These are loaded from the assetManager when the
AssetLinkNode is loaded from a binary file.

@param key the ModelKey to add
jme3-core/src/main/java/com/jme3/scene/AssetLinkNode.java