Credit unit type validation

[rsaksida]

Addresses #33 and #31. Partially addresses #32.

There are two issues:

• the model hallucinating credit unit types
• the model incorrectly detecting the credit unit type

Proposed solution:

• instruct the model to set the credit unit type as "Unknown" when it can't confidently detect it
• validate the output vs the valid credit unit types
• leave it blank in the CSV when it can't be detected