Migrate requirements to pyproject.toml

[mranst]

This PR moves the requirements to pyproject.toml. Github requirements are specified in a separate section of pyproject.toml. This will impact how users install swell, it basically just eliminates the need for separately installing requirements using the --requirements tag. Docs for all install methods have been updated. I've left requirements.txt present with a deprecation warning for the time being. This will give users a chance to move over to the new installation method without disruption before removing it completely.

[shiklomanov-an]

Looks good to me! Probably the sequence of PRs should be:

1. This one
2. Change CI-Workflows to use these build/install commands.
3. Here -- remove requirements.txt.

[mranst]

Looks good to me! Probably the sequence of PRs should be:

1. This one

2. Change CI-Workflows to use these build/install commands.

3. Here -- remove `requirements.txt`.

Thanks, makes sense. The application tests should continue to run fine on develop with how it's done there, but if we switch to the new install commands it won't work for people's branches who haven't caught up yet, so I'm thinking I'll give it a little bit before changing it

[Dooruk]

Looks good! uv pip install -e . improved my life already.

If we are looking into making some changes to CI-workflows, is it worth revisiting:
GEOS-ESM/CI-workflows#11?