Orbitals not showing up

This is in the most recent `master` fb2eee08faca347444dc7bbf85de2d8792ccd6f2 and also MDT. You'll also need to check out the `visualization_json` branch of MDT, or this will throw an exception because of some bad JSON.

To reproduce:
```python
mol = mdt.from_name('benzene')
mol.set_energy_model(mdt.models.CASSCF, active_electrons=6,
active_orbitals=6, state_average=6, basis='sto-3g')
mol.calculate()
mol.draw_orbitals()
```

Then select an orbital from the dropdown list. This produces:

But we'd expect orbitals:

(the UI changes are from upgrading to ipywidgets 6.0beta. Once that project is fully stabilized, we should probably make sure the UI layouts are reasonable)