Fix node removal

Fixes removal of glam node non-existent node from Viewer.  Here's an example
```html
<glam>
	<scene id='scene0'>
		<group y='-1' id='group0'>
			<cube id="texturedcube" x="1.5" y='2' z='-5' depth='.5' height='1.5' ></cube>
		</group>
	</scene>
	<scene id='scene1'>
		<group id="testGroup" x='-3' y='1' z='-5'>
			<cone id="cony" x='1' y='-.5' z='2' rz='90deg'></cone>
			<cylinder id="cylinder2" x='-2'></cylinder>
		</group>
	</scene>
</glam>
```
Say I want to move testGroup to scene0, but scene1 was never parsed by glam.
```javascript
var scene = document.querySelector('#scene0');
var group = document.querySelector('#testGroup');
scene.appendChild(group);
```
This used to fail as it attempted to remove `testGroup.glam.obj` from the `Viewer` in  `glam.Viewer.prototype.removeNode` , where `glam` (and thus `obj`) is undefined as it was never added.