Vocabulary support

[lgessler]

Implement Vocabularies, as specified in the documentation.

[lgessler]

@michaelpginn makes the good point that given their different behaviors, the version of this used for tokens should probably be different from the one used for spans and relations.

[lgessler]

• See data model design doc for details
• We're going to use the full many-to-many-capable approach for tokens for spans and relations as well. Unlikely that a many-to-many structure would be meaningful for spans and relations, but it simplifies development to just reuse the same Vocabulary mechanism for everything
• Core structures:
  • vocab-layer (global): has a name, also layer-type (token/span/relation)
  • vocab-item (global): has a form (for tokens, this is a reference form; for non-token applications this is just the value of e.g. the POS tag); has properties (arbitrary map of KVPs for now, could make this structured in the future to e.g. provide validation)
  • vocab-map (document-level): maps between tokens/spans/relations/... and a vocabulary item. Has members: collection of IDs of linked items
• CRUD: vocab-layer
  • Delete: delete all vocab-items and vocab-maps
• CRUD: vocab-item
  • Delete: delete all vocab-maps matching this vocabulary item
• CRUD: vocab-map
  • Deletion on a token/span/relation: need to do the same thing we did for spans: remove from all vocab-maps, and then delete the vocab-maps if its associated item set is now empty (deletion on token/span/relation layer should already automatically trigger this)