Move `userdata` from `Record` to `RecordInfo`

[pinkwah]

The use-case has been ambiguous, and ERT is mostly using it with ?realization_index=0 to work around the mess.

I propose we move the userdata field from Record to RecordInfo. The API will be the same.

For backward-compatibility, this endpoint should still accept realization_index, but if it is something other than 0 it return HTTP 422 Unprocessable Entity.

[jondequinor]

This is the case where we stuff a list of records into one record with realisation index 0?

Why be backwards compatible? Current ERT use is a hack that is, to my knowledge, not in use—except in testing.

[xjules]

This is the case where we stuff a list of records into one record with realisation index 0?

Not really. The issue being that only realization_index=0 holds the metadata for all the realizations which already makes it erroneous since when writing any realization record (with realization_index>0) thus expects that realization_index=0 exists. Therefore putting metadata into RecordInfo makes perfect sense.

Why be backwards compatible? Current ERT use is a hack that is, to my knowledge, not in use—except in testing.

Agree here. I would say we don't need to be backwards compatible as the API is still being developed...