Import metadata.owl to get "curation status" classes and individuals

Following discussions with @alanruttenberg at [Credible workshop 2014](https://credible.i3s.unice.fr/doku.php?id=2014_workshop), this pull request:
- Import OBO `metadata.owl` in `nidm-results.owl`
- Delete previously manually copied terms and classes for "curation status"
- Update `UpdateTermReadme.py` to deal with curation status terms instead of strings.

This pull request does not induce any change in the data model. Only the way we handle curation status is affected.