text: Title: Example "volume_render" does not work
  Body: I'm trying to run an example [volume_renderer](https://github.com/K3D-tools/K3D-jupyter/blob/master/examples/volume_renderer.ipynb) and I have error. In the case of Binder, module 'nibabel' don't exist. If I try locally with installed 'nibabel' - there is no 3D object, only an empty grid of coordinates is visible.
  