Avoid recursion in the tasks if no particle is active.

Does exactly what it says. We don't want to go through all the cells recursively just to find the handful that need this task to act upon.

Big improvements when added to the kill_drift branch on the EAGLE_25 case.

Reassigned to @pdraper

LGTM!

Here is what it does.

BEFORE:

AFTER:

Sorry for putting more and more pressure on the threadpool parts.

a big fraction of the time spent on one core is the initial setup. I am still unhappy about the reading-in of the data.

Status changed to merged

mentioned in commit 5641fa31