Add project: `apax`: Atomistic learned Potentials in JAX!

[PythonFZ]

What kind of change does this PR introduce?

• [x] Add a project
• [ ] Update a project
• [ ] Remove a project
• [ ] Add or update a category
• [ ] Change configuration
• [ ] Documentation
• [ ] Other, please describe:

Description: apax is a high-performance, extendable package for training of and inference with atomistic neural networks. It implements the Gaussian Moment Neural Network model [2, 3]. It is based on JAX and uses JaxMD as a molecular dynamics engine.

Checklist:

• [x] I have read the CONTRIBUTING guidelines.
• [x] I have not modified the README.md file. Projects are only supposed to be added or updated within the projects.yaml file since the README.md file is automatically generated.

[Irratzo]

Thanks for the addition @PythonFZ! I will add you as co-author with your ORCiD in the next release, see #256. This month, hopefully.

[Irratzo]

@PythonFZ, I added you to CITATION.cff. Hope that's okay, and thanks again.