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 - damn I just noticed these two bullet points. The second is no longer true, right? Shall I re-word to say

```
The proposed ``NDDataBase`` simplifies the current ``NDData`` class to the extreme,
such that it essentially only defines the properties needed for ``NDData``?
```