Refactor splitter.py, add helper functions, ready for FMTM integration

[spwoodcock]

Fixes #1 Fixes #3

I significantly refactored to fix up a few things:

• DB session handling, uses existing session from fastapi, or a single new session.
• DB url is configurable (we can pass in the FMTM DB).
• Added the database models to this repo. I made it so that the tables are created by this repo, splitting done, then tables deleted again. This way we don't need the table models in FMTM either, simplifying the codebase.
• Init the AOI as a geodataframe within the class init (puts the validation in one place).
• More flexible input types, particularly for geojson (can be geojson string, FeatureCollection type, or a geojson file on the filesystem).
• Testing for all split types.
  (I think that was it!)

I also added helper functions for the three splitting types, for more flexible usage.
They are called by the CLI and can be imported for use in say FMTM.

[spwoodcock]

Note to self: I also need to update the CI workflow to use test_compose instead now as this needs an underlying db.