WIP: Sink particles

Hey,

In this merge request, I am implementing the base for the sink particles. I am able to read / write the particles, restart and ... that's all.

I have also copy/paste the Isothermal example in order to run it with sink particles (I am planning to delete it when my work will be done).

mentioned in merge request !1067 (closed)

mentioned in merge request !1068 (merged)

closed