Make sure the self tasks get a drift even when we don't need to split them at the top level.

Why do the sub-cell self tasks need the drift to be there? They don't depend on it (at least they shouldn't), as they drift the particles on the fly.

At the moment my gravity tasks don't trigger a drift of the particles. They need a drift task to exist.

I will need to look back at splittask and very likely make an independent version for the gravity tasks. I did not want to break functionalities for now. Let me make that drift task creation only for gravity sub-types.

OK, good for me then!