Repartition threadpool

Closes #462

See merge request !663

Logger: Add double time in timestamp

See merge request !666

Create a separate MPI operation for the top-level multipole reduction.

Closes #485

See merge request !661

Add Hernquist and NFW potential to SWIFT

See merge request !662

Add debugging check for weights construction that makes sure the old serial code and threadpool are consistent

Also fixes bugs this revealed!

Make mpicollectgroup1_reduce() loop over all the elements as described by the MPI standard.

See merge request !660

When resuming from a check-point file, we do not have the list of local top-level cells for the original drift all. Use all the cells instead.

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.

Move the task plot legend so that it does not overlap the top of the graphic

See merge request !646

Engine cleanup

See merge request !657

Check the return value of the top-level multipole all-reduce.

See merge request !656