Reorganise API & Model modules

[obmarg]

• API modules now have a . between their group & version names.
• Models now live under their associated API module where possible.

[obmarg]

TODOs:

• [x] Update documentation for this change.

• [x] Add section to readme explaining the organisation of modules.

• [ ] Try and look into #20

• [x] In particular need to fix this bit of the docs: it doesn't actually say the whole API name.