SF split: update s->mass_*_min and small fixes

• When you form a star, GEAR's model decreases the mass of the part. Therefore s->min_part_mass needs to be updated.
• When exchanging the particles (in the strays), you can exchange the lightest particle and therefore have a wrong s->min_*_mass. Therefore after the exchange, we need to recompute it.
• In star_formation_copy_properties, I forgot to inverse spawn_part when providing the argument (due to the API change).
• In GEAR's model, I forgot to divide by the number of stars per part.

For s->min_*mass, I think we are using it only for debugging checks. Should I put all the new code between #ifdef SWIFT_DEBUG_CHECKS?

I am still running the SmallCosmoVolume example with the full physics (4 ranks and currently at z=5.5) but I think it should be fine for the end of the simulation.