Support Scatter/Gather operations

[Joroks]

currently only lammps_gather_atoms() is implemented.

however, I'd like to use

• lammps_gather()
• lammps_gather_subset()
• lammps_scatter()
• lammps_scatter_subset()

as well.

If I've understood the C-Library documentation (https://docs.lammps.org/Library_scatter.html#_CPPv420lammps_scatter_atomsPvPKciiPv) correctly,

• lammps_gather_atoms()
• lammps_gather_atoms_subset()
• lammps_scatter_atoms()
• lammps_scatter_atoms_subset()

do essentially the same thing but are more restrictive (i.e. don't work with fix or compute values). So I don't think it would be necessary to implement those?

[vchuravy]

Happy to take a PR for these. I currently am short on time so I don't think I can implement it myself.

We currently don't have a high level interface to scatter:

The only use I see is

LAMMPS.jl/examples/lj_forces.jl

Line 28 in 7201aec

LAMMPS.API.lammps_scatter_atoms(lmp, "x", 1, 3, positions)

You could follow

LAMMPS.jl/src/LAMMPS.jl

Line 364 in 7201aec

function gather_atoms(lmp::LMP, name, T, count)

to add a Julian interface for these functions.

[Joroks]

I'll get to work on it then.

[vchuravy]

Implemented in #43