Each runner now contains a ci_cache and cj_cache.

src/runner.h

@@ -49,8 +49,11 @@ struct runner {
   /*! The engine owing this runner. */
   struct engine *e;
 
-  /*! The particle cache of this runner. */
-  struct cache par_cache;
+  /*! The particle cache of cell ci. */
+  struct cache ci_cache;
+  
+  /*! The particle cache of cell cj. */
+  struct cache cj_cache;
 };
 
 /* Function prototypes. */