Added comment to explain the conversion to N_ngb from wcount in test27.

tests/test27cells.c

@@ -204,6 +204,8 @@ void zero_particle_fields(struct cell *c) {
 void end_calculation(struct cell *c) {
   for (int pid = 0; pid < c->count; pid++) {
     hydro_end_density(&c->parts[pid]);
+
+    /* Recover the common "Neighbour number" definition */
     c->parts[pid].density.wcount *= pow_dimension(c->parts[pid].h);
     c->parts[pid].density.wcount *= kernel_norm;
   }