mixing 2D and 3D objects

[chrishavlin]

With #62, you can now load in a mix of slices and volumes with the caveat that the slices are treated as independent image layers. This means they will not be scaled or oriented in the way the 3D volumes are.

I played around with getting a 2D plane into 3D in a way that the position in 3D can be controlled and the simplest way is to add a singleton 3rd dimension to the image array, which allows specification of all the 3D vectors that control location (the scale, translate, rotation arguments to add_image). see this example for reference.

So it should be possible to orient the 2D planes relative to the 3D scene, but I'm going to leave this to a future release...