Vocabulary/taxonomy improvements

CKAN currently has a basic model for creating a taxonomy, loading terms into it via the API and you can create a form field that allows values from a taxonomy. Terms can be translated.

However at DGU we want to a categorize each dataset using a hierarchical taxonomy (we are thinking COFOG) that has been published online, and terms will be added/removed in time.
- a term needs an ID (e.g. URI) attached
- terms need to reference other terms in the vocabulary (e.g. for a tree hierarchy)
- script that syncs terms from another source (e.g. an online RDF file containing hundreds of terms)

As a related item, it would also be handy to be able to call a service that can auto-categorize a dataset using:
- categorization in another vocabulary (e.g. ONS custom one, or Eurovoc)
- using text analysis and machine learning based on a manually curated training data

We've found people are terrible at selecting categories for their datasets.

The issue names no files, tests, or entry points; start by reviewing CKAN's existing taxonomy model and API, then clarify the requirements for hierarchical terms, external synchronization, and automated categorization. Done would require an agreed scope and design covering the selected taxonomy capabilities and their validation.

Written by the indexing model from the issue text.