New spectral cubes PR

Fixed other commits appearing in the other PR. @Cadair, could you please close the other one?
funny... If I do this with a 3D cube with `None` I get the following:
```python
>>> nn = np.arange(1000).reshape(5,20,10)
>>> nn.shape
 (5, 20, 10)
>>> mm = nn[:, None, None]
>>> mm.shape
 (5, 1, 1, 20, 10)
```