Unify representation of dimensions_values_labels
Currently the data model defines an alternative for the type of the field
As discussed with @MichelJuillard, it seems that the API should return a unified representation of
dimensions_values_labels. But the JSON files stored in Git repositories can keep the dual representation.
To simplify the job of clients (like the UI), the API should only return a list of pairs, so:
- if the type is a list of pairs, return it as-is
- if the type is an object, convert it as a list of pairs with keys ordered lexicographically