Add information to stdout and timestep.txt about what happened in the step (rebuild, repartition, snapshot, ...)

Implement simple thread barriers to be used on systems where the POSIX implementation does not provide them. We default to the POSIX ones when detected at configure time.

Use the threadpool to collect data over the top-level cells at the end of the timestep. Also only loop over the cells that have tasks in their hierarchy.

Instead of listing local cells only, list cells that have a tasks associated with them on the local node.

Fixed the gravity bugs reported in #363. Need to find a good rebuild criterion for gravity-only runs.

Fix to #363. Drifting decision needs to use the same multipole information as the actual tree walk. Also fix some gravity_cache padding values.

There is a problem when the cpu mask is empty and using multiple MPI ranks on a node, in that case all the main threads end up pinned to one core during the reading of the initial condition (and any other tasks done before engine_init is called).

Force compiler to report shadowed variables. Removed all shadowed variables. Added the function scheduler_activate_send() to simplify the unskipping of MPI sends.

Reinstate missing pointer indirection for external gravity dependencies that had disappeared in merge.

Hopefully still quite conservative

Correct use of caches in the self-PP gravity task. Make sure we can run without vectorization. Make sure we can run without hydro.

Correct use of caches in the self-PP gravity task. Make sure we can run without vectorization. Make sure we can run without hydro.