RT gradient + transport tasks

This merge request adds the gradient and transport (hydro) particle interaction loops for the RT scheme. They are tasks themselves are hydro interactions from runner_doiact_hydro.h , so not much new has happened.

WIP status because I need to double-check a thing or two, but all in all it should be fine.

examples/main.c

@@ -579,10 +579,6 @@ int main(int argc, char *argv[]) {
         "chosen\n");
   }
 
-#ifndef GADGET2_SPH
-  /* Temporary, this dependency will be removed later */
-  error("Error: Cannot use radiative transfer without gadget2-sph for now\n");
-#endif
 #ifndef STARS_GEAR
   /* Temporary, this dependency will be removed later */
   error(