RT injection task

The debug version of the RT injection task is ready for review. In this task, we interact stars with hydro particles and distribute the star's photons amongst close-by hydro particles. Hence a neighbour loop with self/pair cell interactions is required. The RT tasks are intended to start after all star related tasks are done (after all, we want new stars to contribute, as well as to take into account the heating of the surrounding gas from the stellar feedback) and once finished, unlocks the timestep task.

This merge request adds the following to the code base:

• the self, pair, sub_self, and sub_pair task prototypes have been written.
• a struct rt_data has been added that extends both hydro and star particles (individually)
• prototypes for RT output writing
• the plot_task_dependencies.py tool has been extended to be able to plot RT tasks.

Future updates of this task will include (but not within scope of this merge request!):

• ability to work with other SPH schemes than Gadget2
• ability to work without star feedback. For that to work, I need to introduce a ghost task first. As of now, I'm relying on the stars_out ghost task to do the trick.
• remove the stars_out -> timestep dependency when running with --radiation.
• everything MPI related.
• everything RT subcycling related.

@lhausammann: Could you have a first look at it before we dump this on Matthieu's desk?