Otherwise until engine_init we see [0000]

Mark tasks in drift2

This "works" as in "I'm going to be now, it runs 100 steps of CosmoVolume without barfing".

@pdraper, if you have any spare cycles, can you run it on anything you've got until it breaks? Otherwise, I'll just keep on hacking at it as soon as I've got another hour or two. Thanks!

See merge request !267

Conflicts:
   src/engine.c

Add cell links for gravity tasks

Add links for gravity tasks so that we can discover these from cells.

Also expunge external_gravity from cell.h.

See merge request !268

Expunge external_gravity from cell.h

 Conflicts:
	src/cell.c
	src/cell.h
	src/space.c

free tid_active when finished

Fix super pointer

Updated the logic setting the `super` pointer and creating the "hierarchical" tasks (i.e. init, kick, ghost, etc.). Changes involve:

 - Getting rid of the `g_super` pointer. We have only one `super` cell per hierarchy.
 - Set the `super` pointer in a new routine.
 - Have only one routine to construct the hierarchical task to avoid overwriting things.
 - Moved the external gravity task to be a self with a new kind of sub-type.

The last item is necessary for the following reason. If we want to run with only external gravity (no hydro, no normal gravity), which is useful to test this aspect individually, we need to create the tasks. Now, we only create the init/kick tasks for super cells, i.e. cells that have at least one self or pair. So I promoted the external_gravity task to be a self with a new sub_type. 

IMO that's more clean now. What do you think ?

Fix #215.

See merge request !259

first try to merge in the scheduler_active stuff from the scheduler_skip branch. currently broken, apparently engine_collect_timestep isn't doing anything.

Parallel space_rebuild()

Looking at some of the scaling results, it turns out that the last remaining significant chunk of non-parallel code is `space_rebuild()` and the majority of the time in there is spent computing the cell index of the particles. 
This can easily done in parallel and on the EAGLE_25 shows significant improvements in the code speed and scalability. Although I should say that this comes from running this on 16 cores only and based on the vTune outputs (which usually match the actual tests).

What do you think ?

See merge request !260

Now that we are skip 1, that cannot be used to pick out the active tasks for the thread info dumps, use task toc=0 instead

Strange but drift doesn't seem to visit all cells to reset this count