Fix issue 1794 monitor_nd atomic listmode

[willend]

It turned out that the cure was to do an "atomic capture" to define a local TRACE copy of the "particle count", to be used everywhere in TRACE.

In a CPU setting where only one thread is active at a time we can of course make do with the usual, global variable on the structure:

  #ifdef OPENACC
  /* For the OPENACC list buffer we need a local copy of the
     atomically updated Neutron_counter - captured below under list mode */
  long long ParticleCount=0;
  #else
  /* On CPU can make do with the global, declared one... */
  #define ParticleCount Vars->Neutron_Counter
  #endif

At the "incrementation-point" we capture atomically from the GPU in OPENACC settings:

        #ifdef OPENACC
        #pragma acc atomic capture
        {
	  ParticleCount=Vars->Neutron_Counter++ ;
        } 
        #else
        ParticleCount++;
	#endif