Draft: RT: Set upper limit for star time step sizes

Add an upper limit for stars based on neighbouring gas particles' time bins. The max bin difference is currently set at 4, but ideally should be researched more closely. Other than that, I'd say this MR is good to go on my end.

src/stars/Basic/stars_part.h

@@ -102,6 +102,9 @@ struct spart {
   /*! Radiative Transfer data */
   struct rt_spart_data rt_data;
 
+  /*! Neighbour RT time bins */
+  timebin_t min_rt_ngb_time_bin;
+
 #ifdef WITH_CSDS
   /* Additional data for the particle csds */
   struct csds_part_data csds_data;