Conflicts:
	examples/main.c
	src/engine.c
	src/engine.h

Use of threads as command-line option.

Made the number of threads a command-line parameter again.

I have restored the `-t` command-line parameter and removed the parameter from all the yaml files. I have also updated the run.sh scripts with a default value of 16. 

That fixes #144 and the branch can be removed if you are happy with the changes. Thanks !

See merge request !144

Root level parts

Handles the case when the top-level cell size increases so that
the cells need to be regenerated and particles re-attached.

The technique is to resample the old cell space and use the node
IDs to populate the new one. This should minimise the need for
particle movement, so should be a little less expensive than a 
full initial partition.

If the resample fails we fall back to a full initial partition.
Could be a better idea to use the same technique as in main.c,
but we don't do that.

Fix for issue #31

See merge request !112

Conflicts:
	examples/main.c
	src/const.h
	src/engine.c
	src/engine.h
	src/space.c
	src/units.c
	src/units.h