Merge branch 'subtasks_only_for_cells_with_parts' into 'master'

Do not recurse to lower cells within the sub-tasks if they don't contain any particles.

This should fix #274. @jwillis could you please check that your setup that crashes works with this version ? 

It builds on your proposed fix but I moved the decision of aborting higher up in the recursion. 

See merge request !326