GEAR sinks: add better time-step and sink merging constraints
In this MR, I added timestep constraints to the sink and made the sink-sink swallow criteria more similar to the gas ones.
- Sinks follow the gas dynamics more closely with a CFL and gas free fall time-step constraints.
- They now have a birth time and thus an age. I implemented time-step constraints based on their age category, similar to stars.
- Dead sinks do not accrete anymore.
- Sink merging criteria are the same as those for gas.