Update "Large-scale simulation planning" section

[fjclark]

Proposed Change

Expand "Large-scale simulation planning" section and ensure that it's coherent.

Possibly add discussion of methods which are not easily characterised as single/ multi e.g. lambda dynamics? Maybe a better distinction is calculations where we are interested in more than two end states.

Proposed Authors and Reviewers

Please volunteer (before the end of January) 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 author: Volunteer(s) wanted
Proposed reviewer: Volunteer(s) wanted