if the threadpool is initialized with a single thread, don't bother spawning a r…

…unner, just run in the threadpool_map callee, which is blocked.

See #252.

See merge request !293

Space recycle

Do the recycling not cell-by-cell, but with lists of cells at a time.

See merge request !291

if the threadpool is initialized with a single thread, don't bother spawning a runner, just run in the threadpool_map callee, which is blocked.

Conflicts:
  src/space.c

This reverts commit 00499932.

Minimize particle movement during repartitioning

Attempt to keep particles on local nodes by comparing the new and old regions and
permuting the solution by picking new regions with the largest number of 
cells in the old regions. Gives an unmoved fraction of around 42% for larger
numbers of ranks (compared to `1/nrank` for the current scheme).

Doesn't make a huge improvement to runtimes, seems to be around 10% on a
repartition step of `EAGLE_25`, and we only do that infrequently.


See merge request !288

Profiler

Summary
-------
- Profiler object that can be used to time functions and write the results to an output file

See merge request !289

Tidy up. Remove commented out code mainly, just output used fraction when verbose, revert changes to main