APE7 - A plan for NDData

This is ready for review. Comments are solicited until **Friday 14th November** after which a vote will be taken on whether to accept or reject this APE.

(note, this has been renamed to APE7 since there is already an APE6 proposed in #7)
@mwcraig - I added this note after comments on the list. Note that there is a difference between allowing the property to be set and to be modified. So one can still do:

```
nd.mask[:,:,:] = False
```

even without a setter. I don't feel strongly about this though, I think that ``wcs`` and ``unit`` should not have setters, but don't see why the other ones couldn't have setters. Maybe @mhvk can comment on this?