Move the MPI type for multipole to a separate file. Create a bit-by-bit XOR reduction operation for multipoles. Utilise that reduction operation when exchanging the top-level multipoles.

Use temporary local copies of the D_xxx terms in the M2L_apply function to help the compiler optimize quantities.

Restore the early exit for cells that are far-away in the long-range force task. Restore the check that all particles interacted with everything in runner_end_force.