Propagating uncertainty between models in EpiNow2

This issue relates to the talk (Community Seminar 2023-03-01 - Adrian Lison - Generative modeling approaches to nowcasting with incomplete line list data) @adrianlison will be giving at the next Seminar.

He has done quite a lot of work looking at the impact of passing estimates between models (as done in EpiNow2 vs joint modelling as done in his work and optionally in epinowcast.