Create restrictions on file fields

[nagem]

Use an enum in our input json schema to restrict values saved to type and modality fields

[nagem]

Also incorporate #210

[nagem]

@gsfr I had written down file type, modality and "instrument" when I think perhaps I meant measurement? Please confirm the fields we want to restrict to a list of approved values when you get a chance.

[gsfr]

Yup. File type and modality are easy. Measurement is harder, because it depends on modality.

[nagem]

Encompassed/replaced by #649