New spectral cubes PR

Fixed other commits appearing in the other PR. @Cadair, could you please close the other one?
That's a numpy thing - using None when slicing adds an axis - so

    >>> mm = nn[:, :, :, None]
    >>> mm.shape
     (5, 20, 10, 1)
    >>> mm = nn[:, None, :, None, None, :, None]
    >>> mm.shape
     (5, 1, 20, 1, 1, 10, 1)

I forgot to include that behavior...