Migrate to pyproject.toml and pydata theme

This MR migrates from setup.py to pyproject.toml and changes the Sphinx theme to pydata

• I have set up the pyproject.toml file so that there are mandatory dependencies (ase, numpy and scipy), as well as test and docs dependencies. Then in the Dockerfile/CI we can use pip install strongcoca[docs,test] to install all the dependencies at once, and we don't have to list them in multiple places.
• The Dockerfile now contains the somewhat hacky instructions to
  1. Copy the code and pyproject.toml to the container image.
  2. Install strongcoca and all dependencies
  3. Uninstall strongcoca (but the dependencies are kept)
  4. Remove the code and pyproject.toml from the container image
• I changed the URL for the webpage to https://strongcoca.materialsmodeling.org/