GEAR sink: add a minimal mass to sink

In this MR, I implemented a minimal mass threshold when sink spawn stars. So, they cannot spawn stars if m_sink - target_mass < sink_minimal_mass. We discussed it with Yves.

The second condition is relevant for low-resolution simulations, where a newborn sink can spawn stars because the gas mass turned into the sink is big. After spawning the stars, we have sink_mass << gas_mass, which gravitational interactions can kick away. Also, gas particles can never be bound to the sink if their mass is >> sink mass.