Use pyproject.toml and src rootdir

[sandorkertesz]

This PR adds the following changes:

• uses pyproject.toml instead of setup.py
• uses the src/earthkit source structure

Please note that temporarily we have this option in pyproject.toml:

[tool.setuptools.packages.find]
include = ["earthkit", "earthkit.*"]
where = ["src/", "./"]

This is to make earthkit components not yet using the new src/earthkit/ source structure work. The downside of it is that we cannot query the version of earthkit itself when earthkit is installed from the local git repo as pip install -e .. However, this is only a concern for developments.

Once all the earthkit components are converted we should use:

[tool.setuptools.packages.find]
include = ["earthkit"]
where = ["src/"]