Update discussion of uncertainty estimation

[fjclark]

Proposed Change

Expand discussion of uncertainty estimation (avoiding topics discussed in the "single calculation" paper).

Proposed Authors and Reviewers

Please confirm (before the end of January) that you are happy to author/ review and state when you're available. Authors, please say roughly when you plan to contribute the changes (no later than the end of May).

Proposed authors: @apbhati
Proposed reviewer: Volunteer wanted

[apbhati1]

I am happy to contribute to this section.