Add Star formation spart struct

I implemented the changes as discussed, this change will add an struct for the star particles specifically for the star formation similar to the chemistry struct, this will make the stars and star formation models more modular and also if we want to change what is stored in the stars related to star formation, than we only need to change the star_formation routine. I updated all the star formation and star schemes:

• EAGLE stars & star formation
• GEAR stars & star formation
• Default stars and none star formation
• Quick Lymann Alpha star formation.

I tested the code with the EAGLE subgrid on the isolated galaxy with feedback and that seems to run to completion. Also the code compiles when I use different subgrid routines and the default ./configure. I am not able to test if the GEAR model works because I don't have crackle (it compiles). Let me know what you think.

@lhausammann this cleans up things a bit. The idea is to store the stars-carried quantities assigned at birth in a small sub-structure in the spart. This touches GEAR. Happy with the changes?

This is something I wanted to do since a long time. Thanks Folkert.

I am happy with the changes.

Great, thanks both.

merged

mentioned in commit e8b4adc6