Separate FAISS index creation from init

[luciaquirke]

• Write a config to the faiss directory
• Separate create from init
• Require sufficient RAM upfront
• Enable calculating full scores with FAISS
  • FAISS is optimized for the TopK use case but it's good for convenience. We probably want to provide a separate utility for calculating full scores straight from the mmap in the long term because building the index seems unlikely to be worth it for just scoring (?)