Add GPU acceleration to the LAMMPS plugin through Kokkos

[anjohan]

This adds pair_style flare/kk for GPU acceleration. It will run on CPU with OpenMP too, but that's slow and unoptimized.

I've also added

• lammps_plugins/README.md.
• lammps_plugins/install.sh which symlinks files into lammps/src[/KOKKOS].
• A link in the Sphinx docs to the above README.
• GitHub Actions tests for LAMMPS with OpenMP through Kokkos (and I changed the build to CMake).

Current restrictions: B2, squared dot product, quadratic cut-off, same cut-off for all pairs, Chebyshev radial basis.