[DRAFT] improved probability management

Collaboration with the [probabilit idea](https://github.com/equinor/probabilit)

Need to better define what do we want there, what kind of functionalities do we want?

Start by reviewing the linked probabilit idea and this issue's open questions about desired functionality. Clarify the intended probability-management scope with maintainers before locating implementation entry points; done means the required functionality and acceptance criteria are defined.

Written by the indexing model from the issue text.