swiftsim

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