Updates to metadata model

[johnglover]

Finalised metadata model requires several changes to our existing model:

• new extra: alternative title
• new extra: identifier
• new extra: interop level
• new extra: version description
• new extra: dataset status
• dataset type: change to fixed vocab
• resource types: prefix with documentation/distribution/etc for clarity, add new resource types
• rename update frequency to accrual periodicity
• add new fixed vocab field: language coverage
• resources: missing issued date - add as new core field called 'created'

Also add functional tests for full metadata model.