Draft: RT: Set upper limit for star time step sizes

Add an upper limit for stars based on neighbouring gas particles' time bins. The max bin difference is currently set at 4, but ideally should be researched more closely. Other than that, I'd say this MR is good to go on my end.

added radiative transfer label

requested review from @zhenxiang

assigned to @matthieu

added 41 commits

• 04ccd784...247f8029 - 37 commits from branch master
• ea7f89a4 - added rt_timestep_data to stars
• 4d193d4a - added collection and resets throughout code
• 48564eec - added actual limitation. Removed time bin struct for stars, one time_bin is enough.
• 3d0c8264 - applied formatting tool

Compare with previous version

mentioned in issue #829

added 41 commits

• 3d0c8264...64593d46 - 37 commits from branch master
• 9a089276 - added rt_timestep_data to stars
• 06cc8f6e - added collection and resets throughout code
• c728f977 - added actual limitation. Removed time bin struct for stars, one time_bin is enough.
• ab8ff9d2 - applied formatting tool

Compare with previous version

Rebased onto master to resolve conflicts in runner_doiact_function_hydro.h.

Code currently doesn't compile due to new changes in master. Should be fixed with !2110 (merged) .