Adopting a code formatting standard

[RandomDefaultUser]

This PR lays a foundation for having a proper code formatting standard within MALA. It employs black

It

• provides a basic configuration of black
  • This also introduces a pyproject.toml file; it currently does not replace the setup.py, but complements it; I would postpone fully replacing it until the upcoming hackathon
• "blackifies" the main code base (including examples and tests)
• Provides a note in the documentation on using black
• Gets rid of unused imports (not strictly a code formatting thing, but while I was at it...)

It does not implement automatic code formatting checks. As mentioned in #501 we can setup a CI for this, but I would also postpone this until the upcoming hackathon, when we wanted to tackle the CI anyway.

Closes #501 (except for the CI, which we will add with the coming CI improvements) and #507.

[RandomDefaultUser]

Ah OK, so I think the pyproject.toml and setup.py did not completely work together yet. But according to this: https://packaging.python.org/en/latest/guides/modernize-setup-py-project/ they should work. I will investigate!

[RandomDefaultUser]

I think I solved the issue with pyproject.toml for now. This is not a permanent solution, as we want to move towards pyproject.toml in full eventually, but it works for now and lets the toml based approach coexist with setup.py
.

[elcorto]

I made small adjustment sugestions in docs/sourc[docs/source/CONTRIBUTE.md, else this looks good vis-à-vis formatting. Thanks. Please see my other comment re. unused imports.

[elcorto]

Regarding #507, I found 3 left-over ones:

$ ruff check --select F401 $(find mala -name "*.py") | grep -v __init__
mala/descriptors/atomic_density.py:136:32: F401 [*] `lammps.lammps` imported but unused
mala/descriptors/bispectrum.py:141:32: F401 [*] `lammps.lammps` imported but unused
mala/descriptors/minterpy_descriptors.py:95:32: F401 [*] `lammps.lammps` imported but unused

All have the form

        # For version compatibility; older lammps versions (the serial version
        # we still use on some machines) have these constants as part of the
        # general LAMMPS import.
        try:
            from lammps import constants as lammps_constants
        except ImportError:
            from lammps import lammps

but the fallback module lammps is never used in the code. Given the comment, I suspect this to work:

        try:
            from lammps import constants as lammps_constants
        except ImportError:
            from lammps import lammps as lammps_constants

[RandomDefaultUser]

You're right, there are three leftovers. I consciously left them in, because I am not entirely sure myself anymore how the old version handled lammps_constants. Looking at your suggestion and reflecting at what happens there made me realize there is no point in being compatible here. A LAMMPS version that old will not work with our current codebase anymore.
I will delete these lines, and then all unused imports should be gone.

[elcorto]

Thanks, looks all good now.