Choice of reference per batch

[arthur-flam]

There are some usability improvement we could do for projects where there are multiple batches of results (e.g. denoise, hdr...):

• Currently, if users select say denoise, then hdr, the reference is still set to denoise. Let's change that! (unless maybe users explicitly selected the reference batch and we can find comparisons...?)
• There is a request for a feature where we can set different references for different batches. E.g. instead of comparing by default vs the latest commit in master, we could compare hdr vs some milestone, and denoise versus another one... Now, would we specify those preferences in batches.yaml, qaboard.yaml? The main concern is remaining faithful to the likely users' intent and doing it, UI-wise, without surprising them...

In term in implementation,

• in the app: we want to make sure selected.reference_batch et selected.reference_commit_id can be undefined, and get the reference we want from the batch.data or project config.
• in the CLI: we need to decide where to read the info and save it.

Bye!
Arthur