Updated the default values of space_subsize_pair space_subsize_self to the...

Updated the default values of space_subsize_pair space_subsize_self to the optimal values found in a parameter search using EAGLE_25 on 16 cosma-5 cores.

src/space.h

@@ -43,8 +43,8 @@ struct cell;
 #define space_cellallocchunk 1000
 #define space_splitsize_default 400
 #define space_maxsize_default 8000000
-#define space_subsize_pair_default 64000000
-#define space_subsize_self_default 16000
+#define space_subsize_pair_default 256000000
+#define space_subsize_self_default 32000
 #define space_maxcount_default 10000
 #define space_max_top_level_cells_default 12
 #define space_stretch 1.10f