Separate handling of reference scenario from scenario in scenarios

[denised]

Just confirmed this with Chad: in normal circumstances, the reference scenario should be fixed across the entire solution. The code design should reflect this, rather than treating them as fully parallel and independent, i.e. there should be one "reference" scenario per solution, and if any data from the reference scenario is copied into other scenarios, it should be as metadata only, not value-setting.

Not essential to implement now, but it would make the code clearer in the future.