print write dump systen init warning only on MPI rank 0

#include "dump.h"

#include "dump_image.h"

#include "atom.h"

#include "comm.h"

#include "group.h"

#include "input.h"

#include "update.h"

  if (strcmp(arg[1],"cfg") == 0)

    ((DumpCFG *) dump)->multifile_override = 1;

  if (update->first_update == 0)

  if ((update->first_update == 0) && (comm->me == 0))

    error->warning(FLERR,"Calling write_dump before a full system init.");

  dump->init();