Can now repartition at any time provided some fixed costs have been set.

Now we have ticks all the time the statistics and fixed costs estimates can be permanently available

Switch costs to time based, rather than some arbitrary scaling, makes comparisons more straight-forward

Conflicts:
	src/partition.c
	src/scheduler.c

Also adds new task dumps with basic stats of the tasks and a .h form suitable for inclusion in partition.c

Fix failure to get initial partition with EAGLE_100 volume.

See merge request !672

Two fixes to the gravity code

Closes #492

See merge request !680

In the long-range gravity task, use the top-level cell for the mesh distance check, not the cell where the task is called.

When linking the hierarchical gravity tasks, do not link to the parents if we are at the super-level.

Move tic/toc out of SWIFT_DEBUG_TASKS sections, but keep the --enable-task-debugging configure option as that also requires another two fields that we don't otherwise need

Switch to only using ticks for repartition weights, avoid the problem of early repartitioning by not doing that, the memory balance initial partition is good enough to start with

Correct the debugging code in the snapshot and snap dumps following the merge of the smarter drift branch.

Add MPI comm for stars.ti_end_min

Closes #493

See merge request !679

Fix a bug in the marktask. Can't call cell_is_active_stars() if we are running with MPI and ext potential.

Implement stars sort

See merge request !665

Use the same naming convention for the hydro sort activation function and the star sort activation function.

Improvements to the drift all

See merge request !678

Doc fixes: quick fix for typos and wrong parameter name in VELOCIraptor dox

See merge request !677

When repartitioning, no need to verify that the multipoles were drifted. We are going to free them anyway.