Fix the time-step debugging check for the rare case where a BH has swallowed the particle with the smallest time-bin in a cell.

Only check the correctness of hydro time-steps in cell_check_timesteps() if there are hydro particles in the cell.

Need to distinguish the kicks in the timestep_limiter as some particles may be kicked anyway by the kick1 task.

Make the new task synchronize the particles hit by feedback and force them to recompute a time-step. Propagate the time-step upwards.

Better level for the time-step limiter task to make sure the gravity and hydro steps are synchronized.