ontology table changes

[adamrp]

• version column will be removed
• ontology column will have a unique constraint (thus allowing it to work with convert_to/from_id)
• when metadata is loaded, terms will be checked to make sure they exist in the ontology specified by the user
• no foreign keys will be used right now, however, since ontologies can be updated and terms removed (which would invalidate any studies using those terms). Handling this properly necessitates a UI that supports disambiguation by the user, which we are not implementing for pre-alpha milestone.
• These structures do not support updating ontology versions, so they need to be rethought to do so.

[adamrp]

Also, Many of these concerns were raised by @wasade

[adamrp]

Fixed by #330

[adamrp]

Actually, leaving this open since there are significant improvements that should be made in the future (see original statement in the issue)