Sink formation check

In this merge request, I am implementing the neighbors loop for the sink formation. I am planning to form a sink particle only if it is not a neighbor of an existing sink particle.

I have tested the code for EAGLE 6 with MPI and full physics, everything seems fine.

In order to check my neighbors loop, I have removed the engine_drift_all before a snapshot and run the isolated galaxy example with some sink formation and a single time step for all the particles (I need to ensure that the neighbors loop is done with the position written in the snapshot). Once enough particle are formed, I stopped the simulation and compared the number of neighbors found in swift and with a simple python script on the snapshot. In the snapshot, 1942 sink particles are present. 1941 had the correct number of neighbors and 1 was formed in the last time step (thus the number of neighbors was set to 0).