fix: removing remaining curation status as strings

Following re-use of OBI terms for curation status (#199), #202 and #204
had not been updated before merge.

This commit remove remaining manually defined curation status newly
introduced in #202 and #204.
Merge pull request #206 from cmaumet/fixRemainingCurationStatus

fix: removing remaining curation status as strings
Import metadata.owl and delete manually created curation status
Create OwlReader class and Constants to be imported
Create separate files for manually written part of nidm-results spec
Script to write out nidm-results specs based on owl file
Include used_by and generated_by + ranges
Remove head file (to be replaced by nidm-results.html)
nidm-results_head.html made out of nidm-results.html
Merge section 2 and section 4
Document classes
Add used and generated entities for all activities

(Get closer to initial manually-generated text).
Fix: please send comment (as suggested by @nicholst)
Merge pull request #205 from cmaumet/specsFromOwl

Automatic creation of NIDM-Results specification from nidm-results.owl file (now in Python)
Definition of "ClusterDefinitionCriteria"
Suggestion for related definitions
connected18In3D -> voxel18Connected
Definitions related to Voxel and Pixel connectivity

as discussed in https://github.com/incf-nidash/nidm/pull/201
Update terms README
Update Prov store links