chore: using poetry for dependency management

[chanind]

This PR sets up Poetry for dependency management.

In the process of adding this, this PR does the following:

• removes pylint from requirements.txt and replaces it with flake8, since all the linting in CI uses flake8.
• adds isort since it's referenced in the .vscode config, but seemed to be absent from the actual requirements
• updates the Github Actions config to install via Poetry, and also check black and isort formatting
• updates the Makefile to use Poetry, and fills in the make format and make check-format commands
• runs black and isort, which resulted in a lot of reformatting
• Updates the README to reference Poetry

This PR leaves the following out:

• Leaves requirements.txt in case users are already relying on this to install dependencies. This can be removed if needed (and probably should be at some point)
• Does not add the poetry.lock file to this repo. Since this will be eventually published as a library, it's best not to include the lockfile in the repo IMO.
• Does not move the main sae_training and sae_analysis to a common subfolder. This will need to be done before publishing this as a PyPI module.

[jbloomAus]

Thanks, this is really great work!