Optimization/Rewrite of Radiation Controller

* The performance of the radiation controller as-is was not fast enough for inclusion in production servers, but it has some nice featuers, so rewrote it to be more performant.
* Instead of storing the radiation strength for every turf, we only store the sources of radiation, and calculate the strength only for mobs who might be in range.
   * Old method was ray-tracing to every turf in range whether anything was there to be irradiated or not.  Could be hundreds of turfs.  New method only lazily calcualtes strength at a turf if we actually need to know it.   Often times this is zero turfs if nobody is standing in engineering.
  * Removed the automatic processing of objects with "rad_power" set.  Objects are responsible for calling the repository to create/update their radiation sources.   Saves some extra overhead that in practice was redundant with other process controllers.
  * Also tweaked to be more respectful of qdel'd objects and added some comments.

code/modules/mining/mine_turfs.dm

@@ -559,4 +559,4 @@ var/list/mining_overlay_cache = list()
 	if(mineral_name && (mineral_name in ore_data))
 		mineral = ore_data[mineral_name]
 		UpdateMineral()
-	update_icon()
+	update_icon()