In stats_collect_part_mapper in statistics.c, we don't collect gpe if gpart does not exist



See merge request !278

Remove the fixdt policy. fixdt can be achieved by setting dt_min and dt_max to the same value.



See merge request !277

No task mask

Implementation of #235. There is no such thing as a task mask any more. All tasks in the scheduler get enqueued and executed provided they have not had their skip flag set.

For the special case of the 0th time-step (where we don't compute forces nor kick) there is an extra function in the engine that will set these tasks to be skipped.

See merge request !276

Conflicts:
  src/scheduler.c

New h max definition

Since `h_max` was always an annoyance, here is a better definition. 

There is a new parameter now called `max_top_level_cells` that allows you to set the (cube root of the) maximal number of top-level cells you want. So instead of referring to it as a maximal smoothing length you directly give the number of cells.
Of course if the physics requires larger cells, the parameter is ignored. This default to 12 if unspecified, giving 1728 top-level cells. 

The only reason you would want more top-level cells is for simulations on large systems to obtain a finer MPI decomposition.

I also force a rebuild after time-step 0 now to make sure we have good grid and set of tasks even if the original mesh was based on garbage smoothing lengths values entered by the user.

What do you think ?

See merge request !275

Fixed a bug (reported by Stefan) that was leading to the creation of external gravity pair tasks in scheduler_splittasks()

Safer versions of the error() and message() macros that can be used in if/else statements without braces.

Lean tasks

First step in cleaning up our tasks, remove the `tic`, `toc`, and `rid` fields, i.e. make them optional for debugging.

These values are currently only being used for:

 * Task reweighting with wall-clock values, which is dubious at best and has been removed for now,
 * Repartitioning, which can also be done with the pre-computed task cost estimates.
 * Some error checking in the scheduler, but it's all cases that would cause failures elsewhere, so they won't be missed,
 * Generating task plots, which is still possible if configured with `--enable-task-debugging`.

See merge request !273