cell: move hydro.end_force to end_force

I realized that end_force is not only for hydro but grav too. Therefore I moved the task end_force from hydro to the "root".

Issue written in #474 (closed)

src/cell.c

@@ -2375,7 +2375,7 @@ int cell_unskip_hydro_tasks(struct cell *c, struct scheduler *s) {
     if (c->kick1 != NULL) scheduler_activate(s, c->kick1);
     if (c->kick2 != NULL) scheduler_activate(s, c->kick2);
     if (c->timestep != NULL) scheduler_activate(s, c->timestep);
-    if (c->hydro.end_force != NULL) scheduler_activate(s, c->hydro.end_force);
+    if (c->end_force != NULL) scheduler_activate(s, c->end_force);
     if (c->hydro.cooling != NULL) scheduler_activate(s, c->hydro.cooling);
     if (c->sourceterms != NULL) scheduler_activate(s, c->sourceterms);
   }
@@ -2521,7 +2521,7 @@ int cell_unskip_gravity_tasks(struct cell *c, struct scheduler *s) {
     if (c->kick1 != NULL) scheduler_activate(s, c->kick1);
     if (c->kick2 != NULL) scheduler_activate(s, c->kick2);
     if (c->timestep != NULL) scheduler_activate(s, c->timestep);
-    if (c->hydro.end_force != NULL) scheduler_activate(s, c->hydro.end_force);
+    if (c->end_force != NULL) scheduler_activate(s, c->end_force);
     if ((e->policy & engine_policy_cooling) && c->hydro.cooling != NULL)
       scheduler_activate(s, c->hydro.cooling);
     if (c->grav.down != NULL) scheduler_activate(s, c->grav.down);