Use the same naming convention for the hydro sort activation function and the star sort activation function.

Matthieu Schaller authored Nov 08, 2018 and Matthieu Schaller committed Nov 10, 2018

More uniform way of looping over pairs of top-level cells for the proxy construction, gravity pairs construction and hydro pairs construction.

Replace the absolute depth criterion for gravity tasks with a relative criterion based on the local depth of the tree.

Fixes and documentation of a few more places where the implementation will need to be updated in the future.