Enum for "format" prop in `DurationDatatypeType`

[nsenave]

cf.

Pogues-Model/src/main/resources/xsd/Questionnaire.xsd

Line 936 in a5046ef

<xs:element name="Format" type="xs:token" minOccurs="0">

This format attribute has some specific valid values in Pogues use-case:

• "PnYnM" for a year/month duration
• "PTnHnM" for a hour/minute duration

An enum would be preferrable.

---

Note (for the record): this comes from some old xml standard: https://datypic.com/sc/xsd/t-xsd_duration.html

An other option would be to implement validation methods for this prop. But I think that would be overkill four our use-cases.