Whitelist options for data.unit

[tnatt]

The data.unit field is only defined as a str which makes it difficult to place logic upon.

We should try to make the options for this field whitelisted.

From querying sumo to find what options have been used for this field:
{'', None, 'm', 'ms', 'm3', 'bar', 'Dm', 'fraction', 'mD', 'none', 'unitless'}