Add and display curation status for terms

To support work on the terms, in this pull request, we:
- Add a curation status to each term (re-using [OBI "curation status specification"](http://purl.obolibrary.org/obo/IAO_0000078), taking values "Uncurated", "Metadata incomplete", "Pending Final Vetting", "Requires discussion", "Ready for release"... )
- Move the `nidm-results.owl` file in a sub-directory named `terms`
- Create a README file listing the terms needing revision by curation status.
- Introduce various small fixes in `nidm-results.owl`

A preview of the term listing displayed in the README is available [here](https://github.com/cmaumet/nidm/tree/termsReview/nidm/nidm-results/terms).

I think including a curation status as proposed could help us in determining which definitions we need to focus on. Please let me know what you think. Any feedback is very welcome!