Add pip packaging (scikit-build-core) and an ASE neighbour-list plugin

[jameskermode]

Summary

Makes matscipy-neighbours pip-installable and registers its neighbour list as an ASE neighbour-list backend, so:

pip install .          # builds the C extension into a wheel

from ase.neighborlist import get_neighbor_list, available_neighbor_list_backends
available_neighbor_list_backends()        # {'ase': True, 'matscipy-neighbours': True, ...}
nl = get_neighbor_list("matscipy-neighbours")
i, j, d, D, S = nl("ijdDS", atoms, 5.0)

Selection is explicit and never automatic — this only makes the backend available under the name matscipy-neighbours.

What's here

• pyproject.toml (new) — a scikit-build-core build backend driving the existing CMake. Builds a CPU-only wheel by default (BUILD_TESTING/BUILD_BENCHMARKS/BUILD_EXAMPLES off, so the GoogleTest fetch is skipped); GPU stays an explicit opt-in (-C cmake.define.ENABLE_CUDA=ON / ENABLE_HIP=ON). Declares the ase.plugins entry point. ase is an optional extra.
• language_bindings/python/CMakeLists.txt — adds an install(TARGETS _matscipy_neighbours … DESTINATION matscipy_neighbours) rule so the extension lands inside the package in the wheel; the existing relative-import fallback in neighbours.py (from . import _matscipy_neighbours) then resolves with no source change.
• language_bindings/python/matscipy_neighbours/_ase_plugin.py (new) — a thin adapter matching ASE's NeighborListFunction contract. neighbour_list already returns the same (i, j, d, D, S) letters and D = r[j]-r[i]+S@cell; the adapter only adds the self_interaction keyword, rejecting self_interaction=True (unsupported by the kernel) rather than silently differing.

Notes

• Targets ASE's in-flight v4 plugin system (the ase.plugins entry-point group + NeighborListPlugin). The __ase_plugins__ registration is guarded — on an ASE without that API it registers nothing instead of breaking plugin discovery — so this is safe to merge ahead of the ASE side landing, and a no-op for current released ASE.
• version = "0.1.0" is a static placeholder (the repo has no version metadata today); maintainers may prefer a dynamic/scm version.

Validation

Built and installed with pip install . (cp312 wheel, extension installed inside the package). Against an ASE branch carrying the v4 NeighborListPlugin: matscipy-neighbours edge sets are byte-identical to ASE's reference backend across fcc / hcp / mixed-pbc slab systems (cutoffs 3 and 5 Å; D agrees to ~1e-15). In a neighbour-list backend benchmark it was the fastest CPU backend measured (e.g. ~3× faster than matscipy.neighbours and ~55× faster than ASE's default at 108k atoms) with the lowest peak memory.

🤖 Generated with Claude Code

[pastewka]

Thanks @jameskermode!