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.
`Projections` aren't really intended to be created outside of a `SpectralCube` context... but, fair enough, it's low-cost.