TODO

- finish adding support for all tags in a Product record
  - contributor still needs some obscure tags added
- add a new node type for dates that should be in the form YYYYMMDD
- allow symbols to be used instead of numeric codes
  - contribution type, id type, etc
- validations
  - hard stuff from the spec
    - 2 digit numbers, 8 digit dates, char limits on text fields
  - code lists
    - only values in code lists allowed in relevant fields
  - regional requirements
    - titlepage minimum set of tags
    - APA (if different to titlepage)
- documentation