Add un00 units

[rerpha]

Description of Work

Add a description of the changes here. The aim is provide information to help the approvers review and approve the PR.

Issue

If there is an associated issue, write 'Closes #XXX'

Developer Checklist

• If there are new schema in this PR I have added them to the list in README.md
• If there are breaking changes to a schema, I have used a new file identifier and updated the list in README.md
• There is some documentation here or in the flat buffer file on the use case for this data, including which component is intended to send the data and/or which is the intended receiver.

Approval Criteria

This PR should not be merged until the ECDC Group Leader (acting or permanent) has given their explicit approval in the comments section.
SCIPP/DRAM should also be consulted on changes which may affect them.

[rerpha]

merging as our forwarder relies on this.