Return moments as HDU objects or CubeSlice objects

At the moment, having to type:

```
image = s2.moment(0).value
fits.writeto('moment0.fits', image, clobber=True)
```

is a pain, especially the 'value' part. Of course, Astropy shouldn't require the value to be present, but we could also make it that one could do ``image.write(...)``.