Only add a j-particle when it has mass

j-particles are the particles that exert gravity. If they are massless, they should not become a j-particle.
When deploying PH4 with a CPU backend, this is taken care of in line 207 of idata.cc in "get_partial_acc_and_jerk":
"
		// Skip particles with zero mass.
		if (jdat->mass[j] > _TINY_){
"
However, this is not the best place to do this, it would be better to not create the j-particle at all.
For both the CPU and GPU backends this commit fixes this.
This means that the conditional in line 208 of idata.cc has now become redundant.

Author: HannoSpreeuw

src/amuse_ph4/src/jdata.cc

@@ -139,6 +139,14 @@ int jdata::add_particle(real pmass, real pradius,
 			real dt)		// default = -1
 {
     const char *in_function = "jdata::add_particle";
+
+    if (pmass <= _TINY_) {
+        if (mpi_rank == 0) {
+            cerr << "Warning: ignoring particle with mass="
+                 << pmass << endl;
+        }
+        return -1;
+    }
     if (DEBUG > 2 && mpi_rank == 0) PRL(in_function);
 
     if (nj >= njbuf) {