Store the FOF linking length in the space and store the particle offset into...

Store the FOF linking length in the space and store the particle offset into the global pid array in the gpart.

src/gravity/Default/gravity_part.h

@@ -50,6 +50,8 @@ struct gpart {
   /*! Type of the #gpart (DM, gas, star, ...) */
   enum part_type type;
 
+  size_t offset;
+
 #ifdef SWIFT_DEBUG_CHECKS
 
   /* Numer of gparts this gpart interacted with */

src/space.h

@@ -155,6 +155,9 @@ struct space {
   /*! The associated engine. */
   struct engine *e;
 
+  /*! The FOF linking length squared. */
+  double l_x2;
+
 #ifdef WITH_MPI
 
   /*! Buffers for parts that we will receive from foreign cells. */