Add "Prerequisites and scope" section

[fjclark]

Proposed Change

Add "Prerequisites and scope" section, clearly delineating the separation between the first (single calculation) and second (multiple calculations) papers.

It would be good to set this out early as clarification for authors of later sections - are any members of the steering group happy to do this @ppxasjsm @JenkeScheen @mrshirts @egallicc @vgapsys?

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: @ppxasjsm @JenkeScheen @mrshirts @egallicc @vgapsys?
Proposed reviewer: Volunteer wanted