Space rebuild

Implements #243. space_split with bucketsort instead of quicksort. Reduce the amount of swaps needed to sort the particles into the correct progeny, should be faster and scale better.

See merge request !284

Gravity only tests

Added the configuration command line and CFLAGS/CXXFLAGS to the snapshots and greeting message.

Does what it says on the tin. Just wanted to run it pass you in case you have a better way of doing this. 

Thanks ! Branch can be removed.

See merge request !285

Cooling halo updates

Small update to README for generating videos from snapshots and update to `test_energy` script to be able to run with any number of snapshots.

See merge request !286

Reverted to old space_recycle and space_regrid versions and bug fix in space_rebuild to use nr_gparts instead of nr_parts when extracting cell counts from sorted indices

Scheduler activate root

Re-set the task waits on activation, saves a loop over the tasks in `scheduler_start`.

See merge request !282

Added debugging checks to the pair and self tasks to make sure cells have been drifted to this point before they get updated.

Added debugging checks to the pair and self tasks to make sure cells have been drifted to this point before they get updated.

Added an (expensive) debugging check to verify that all cells have been drifted to the current point in time before a rebuild or a repartition.

Add -funroll-loops as a standard GCC optimization

SWIFT is faster with this.

Fixes #244.

See merge request !283

SWIFT is faster with this

Improve readability of active/inactive condition

This, as is, is rather minor but it's on the way towards improving the time-stepping. 

I have replaced all explicit checks that a particle or cell is active by calls to inlined functions. No change in functionality thus far.

The plan is to slowly introduce a better time-stepping mechanism where the particles carry less information (lower footprint) and make the whole time-step limiter problem easier. With this merge request in then I only need to make the changes in one single location (file active.h) and not everywhere. 

See merge request !279