Simplify the gravity potential calculations for M2L and M2P since the softened case should never take place any more. REplace the calculation by an error message.

Improve the M2P criterion to make sure we don't interact a particle and a multipole if the distance between them is less than the sofetning length (spline truncation radius)

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.