Support simultaneous supersession + clarification in the same proposal

[strogonoff]

This might require slightly altering our data model in order to accommodate multiple proposals for the same item.

@ronaldtse does 19135-1 say anything about multiple changes to the same item within the same [group] proposal? I’d cc Mike but I think we should move this extension to isogr organization for that🤔

[strogonoff]

There are a couple of ways of going about this change.

1. We can support multiple changes for a single item in the same proposal. So there would be a separate change to supersede, and a separate change to clarify.
2. We can support a new change type, such as supersede-with-clarification, which would behave like clarification (item data would be changed) and supersession at the same time.
3. We can make supersession equivalent to clarification, in that we would allow item changes alongside supersession.
4. We can do nothing, and require a clarification to be submitted in one proposal, and supersession to be submitted in a separate proposal if needed. I think this is most in line with the previous version of 19135-1, though I may be mistaken…

Way (1) requires a change in how proposals are structured (meaning old proposals will become invalid or have to be migrated), since we currently key proposed changes by item ID so that there can be no more than a single change proposed per item in one proposal. However, there’s also #70 which we’d need to address eventually, so going this way may be inevitable.

Ways (2) and (3) add complexity to CR workflow. Currently, the only change that can cause item data to be modified is clarification. Other changes modify item status or superseding relationship. It would require a refactor (possibly justified) but also would muddy the waters and make clean change management interface more difficult to achieve.