Pympdata

- scenarios subsystem
- `MPIPolar` boundary condition class
- spherical test case based on [Williamson & Rasch 1989](https://doi.org/10.1175/1520-0493(1989)117%3C0102:TDSLTW%3E2.0.CO;2) (incl. README animations, for now `n_iters=1` only due to PyMPDATA limitations)
- optional dependencies in `pyproject.toml`
- dependency diagram in README (using mermaid)

kudos Delcior

- avoiding buffer allocations in each time step in MPI boundary conditions (using new buffer API in [PyMPDATA 1.0.11](https://github.com/open-atmos/PyMPDATA/releases/tag/v1.0.11))

kudos Delcior

- support for halo-filling when using non-oscillatory option of MPDATA (complex type for min/max pairs)
- animations in README generated through artefact/tip-release uploads on CI

Kudos Delcior

- initial release (after renaming the project to PyMPDATA-MPI)