Allow Extra Fields leads to conflicting data

[saskiad]

Describe the bug
A clear and concise description of what the bug is.

The procedures record for subject = 740955, the specimen procedures have an extra field called "subject_id". This is not part of the schema. For some reason (unrelated to this bug, but something else to fix), these records have an id in this field that does not match the subject_id for the procedures class. This creates problem

If we want to allow users to add arbitrary fields, there needs to be a mechanism that prevents them from adding fields that are present in other places. (e.g. the subject_id is a procedure class field, not a specimen_procedure field).
There are other reasons I'm reluctant to allow extra fields - namely that users will be careless in naming, ignore existing fields but adding other ones, not providing units, etc etc.

I think we should consider not allowing extra fields ...