Fixes to allow the code to run with the EAGLE model spawning stars

Changes:

• When reordering the extra gpart, also take care of potential links with BHs
• The SF can also change the number of gpart. We must thus also exchange the number of gpart created with the other nodes at the level where gravity comms are and not just at the hydro super level.
• Modify the SF task to be able to create an arbitrary number of stars with or without converting the gas particle.
• Modify the EAGLE SF model to allow for the creation of more than 1 star particle / SF event.

Todo:

• Clean-up debugging messages
• Activate the extra comm only if a scheme uses particle spawning? --> Can't do that as it's decided on a particle-by-particle basis in GEAR.
• Make the number of stars formed by the EAGLE model a runtime parameter?
• Not break the GEAR model that spaws but lets the gas particle survive.