Draft: MPI for sink particles

Done so far:

• Sink particle exchange with the proxies (proxy.h/.c, engine_strays.c, etc.)
• Tasks implementation
• cell_unskip is up to date, but not engine_marktasks

To do:

• Clean up debugging messages (some may be worth to be kept)
• Clean up/update comments
• Check some comments I left alongside the black holes when I found weird things/potential bugs.
• Add !1938 (merged) for sink particles
• I disabled calls to gravity functions in GEAR sink sink_iact.h. Move these to some sink_data/sink_merger attribute (similar to potential) to avoid MPI issues when the cells are foreign.
• Check compilation with Default sink
• Take into account the fact that we can use SF and SF_sink at the same time --> use a different sf_counts for SF_sink

Documentation:

• Add a list of MPI-related files
• Update the sink doc to say where we can call gpart and when we cannot to avoid MPI issues.

src/sink/GEAR/sink.h

@@ -171,7 +171,8 @@ __attribute__((always_inline)) INLINE static void sink_init_part(
   cpd->E_rot_neighbours[0] = 0.f;
   cpd->E_rot_neighbours[1] = 0.f;
   cpd->E_rot_neighbours[2] = 0.f;
-  cpd->potential = 0.f;
+  /* cpd->potential = 0.f; */ /* The value was given by
+     sink_store_potential_in_part() */
   cpd->E_mec_bound = 0.f; /* Gravitationally bound particles will have
                              E_mec_bound < 0. This is checked before comparing
                              any other value with this one. So no need to put
@@ -643,6 +644,10 @@ INLINE static int sink_spawn_star(struct sink* sink, const struct engine* e,
                                   const int with_cosmology,
                                   const struct phys_const* phys_const,
                                   const struct unit_system* restrict us) {
+  /* Exit if we have disabled SF */
+  if (sink_props->disable_star_formation) {
+    return 0;
+  }
 
   if (sink->mass > sink->target_mass_Msun * phys_const->const_solar_mass)
     return 1;