Sink formation

In this merge request, I am implementing the sink formation task. This is a task dealing with a single particle at a time (exactly like the star formation). I will need to add an extra loop before in order to ensure that a sink particle is not formed close to another one.

As I am able to form sink particles, I do not need to modify the initial condition for the isolated galaxy anymore. I have removed makeIC.py.

I still need to review the code by myself and rebase the code on the master once the other merge request is merged, but everything should be ready tomorrow.