Allow 1D Spectral Slices

Allow slices of the form:
```
cube[:, y, x]
```

These will return a Quantity subclass similar to that provided by `Projection` and `Slice`.  This is the simplest possible 1D spectrum.  It will include a WCS object and units and can be written to disk.

1D slices in the other directions still are not possible because WCS won't accept unmatched celestial axes.
might as well wrap `value = np.asarray(value)`, to catch lists etc