Reorganise repo

[rchan26]

• [x] Remove data files (we now use data readers for most data sources anyway, so not necessary)
• [x] Rename models/ to notebooks/
• [x] Add reginald directory to store source code

[rchan26]

Will follow this procedure to remove any private data: https://docs.github.com/en/authentication/keeping-your-account-and-data-secure/removing-sensitive-data-from-a-repository

Will do this after merging some other branches as it seems easier if there fewer to rebase.