setup multiple stars

[danieljprice]

Type of PR:
new setup

Description:

• In setup_binary, can make both stars either a sink particle or a gaseous sphere from a stellar profile
• code calls set_star multiple times to add stars
• second and subsequent stars automatically determine the particle number given the resolution in the first star
• same code called from setup_star
• relaxation works to set-and-relax one star after the other
• added new set_units module to simplify read/write of units information from .setup files

Caveats:

• during the relaxation procedure, dump files generated for the second star are currently artificially large (contains the first star but with masked particles)

Testing:
can successfully setup-and-relax two stars and place them into orbit around each other. Tested with polytopes and MESA profiles, would be good to add automated test for this

Did you run the bots? yes

Did you update relevant documentation in the docs directory? yes