GEARRT: conservative flux exchanges

This MR does the same as !1411 (merged) did for Gizmo hydrodynamics: Change the RT interactions and flux exchanges to conservative ones.

Some additional attention was necessary for the RT sorts due to the sub-cycling.

This passes my tests, and I ran a couple of thousand (hundred) steps of EAGLE_12 (EAGLE_25) successfully (until something went wrong with hydro, which is a different problem.) Feel free to stress test this too if you so desire. There is a small catch though: The way to "chose" whether an RT scheme runs with symmetric MPI comms or not is a bit hidden. The MPI_SYMMETRIC_FORCE_INTERACTION macro is defined in src/part.h only when a flavor of Gizmo is selected as the hydrodynamics method. This remains unchanged: The RT will also only use symmetric MPI comms if a gizmo hydrodynamics flavor is selected.

As such, any RT scheme that runs on top of Gizmo hydrodynamics will also have symmetric flux exchanges. And vice versa: Any RT scheme running on top of SPH flavors will have SPH-like "force" loops.

@yuyttenhove , do you perhaps want to have a look at this MR too before the merge?

src/engine.c

@@ -1825,7 +1825,6 @@ void engine_run_rt_sub_cycles(struct engine *e) {
    * so we need to do one less than that. */
   for (int sub_cycle = 1; sub_cycle < nr_rt_cycles; ++sub_cycle) {
 
-    message("------------------------------------ start cycle %d", sub_cycle);
     e->rt_updates = 0ll;
     integertime_t ti_subcycle_old = e->ti_current_subcycle;
     e->ti_current_subcycle = e->ti_current + sub_cycle * rt_step_size;
@@ -1851,8 +1850,6 @@ void engine_run_rt_sub_cycles(struct engine *e) {
           e->step, sub_cycle, time, dt_subcycle, e->min_active_bin_subcycle,
           e->max_active_bin_subcycle, e->rt_updates);
     }
-
-    message("------------------------------------ end cycle %d", sub_cycle);
   }
 
   /* Once we're done, clean up after ourselves */
@@ -2220,8 +2217,6 @@ void engine_init_particles(struct engine *e, int flag_entropy_ICs,
  * @return Should the run stop after this step?
  */
 int engine_step(struct engine *e) {
-  message("==================================== start step %d", e->step + 1);
-  fflush(stdout);
 
   TIMER_TIC2;
 
@@ -2695,8 +2690,6 @@ int engine_step(struct engine *e) {
   /* Time in ticks at the end of this step. */
   e->toc_step = getticks();
 
-  message("==================================== end step %d", e->step);
-
   return force_stop;
 }