Restructure repo

[LorenzLamm]

This PR restructures the repository s.t. it is pip-installable in order to make deployment to pypi possible.

Main changes:

• restructure into smaller submodules (particularly extract utils & expose "slabify" function that can be imported into other programs
• add submodules & init files
• add pyproject.toml file for pip installation
• update readme with installation instructions

After installation with pip install ., setting the path manually is not necessary anymore. The command "slabify" automatically points to the CLI.

[LorenzLamm]

PR against wrong branch