Sink merger

I have implemented the merger of two sink particles:

I realized that my logic for the activation of sink's tasks and my interaction loop for the computation of the sink formation were not correct. As I need the dependencies between the compute_formation and sink_merger, I am using the sink_formation task as a ghost. Therefore it is activated even if there is no sink formation (in order to have the dependencies for the sink mergers).

So my logic is now: If active (hydro or sink), I activate everything for the self. For the pair, I drift the sink of both cells (for the merger) and the part only in the active cell (for the compute_formation). For the sink_formation task, I activate it for both cells.

The logic for the interaction:

• Compute formation: interact all the active hydro particles with the (active and non active) sink particles.
• Sink merger: interact all the active sink particles with the (active and non active) sink particles.

My next step is to implement the accretion function and then I will be done with adding new tasks (still a lot of work, but the most important one will be done).