Merge branch 'super_for_gravity' into 'master'

cell->super for gravity-related business

I think we need to do this to make sure the gravity tasks are hooked at the right place. Given that the particles and g-particles can be distributed in a different fashion, we should not use the same `super` pointer as tasks might be defined at a different level in the tree. 

What do you think ?

See merge request !226