Implement next part of equivalence of AMUSE MR1218

The first part of the code changes equivalent to AMUSE MR1218 have been added in the previous commit 13da4fe.
1) We increased the default value of "nj_max = 16384", which is way too low for many simulations. Instead, we set it to 2^18. The current default in Sapporo Light is 2^17.
2) Also, we added extra info to the error message if "address > nj_max".

Author: HannoSpreeuw

src/interfaces/sapporoG6lib.cpp

@@ -147,8 +147,11 @@ extern "C" {
 //     return grav.calc_lasthalf2(*cluster_id, *nj, *ni,
 // 			       index, xi, vi, *eps2, h2, acc, jerk, pot, inn);
   }
-  
-    
+
+  int g6_get_nj_max_() {
+    return grav.get_nj_max();
+  }
+     
   void get_j_part_data(int addr, int nj,
                          double *pos, 
                          double *vel, 

src/sapporohostclass.cpp

@@ -159,7 +159,7 @@ int sapporo::open(std::string kernelFile, int *devices,
 
     //Allocate initial memory for 16k particles per device
     sapdevice->allocateMemory(16384, get_n_pipes());
-    nj_max = 16384;    
+    nj_max = 262144; //Initial max number of j-particles, can be increased if needed
   }//end pragma omp parallel
 
   //Used to store j-memory particle counters
@@ -461,7 +461,8 @@ int sapporo::set_j_particle(int    address,
     //Extra check, if we are still outside nj_max, we quit since particles are not
     //nicely send in order
     if (address >= nj_max) {
-      fprintf(stderr, "Increasing nj_max was not enough! Send particles in order to the library! Exit\n");
+      fprintf(stderr, "Increasing nj_max = %d was not enough, address = %d\n", nj_max, address);
+      fprintf(stderr, "Send particles in order to the library! Exit\n");
       exit(-1);
     }
   }