update output formatting

    MPI_Allreduce(&n, &size_buf_all, 1, MPI_INT, MPI_SUM, world);

    if (me == 0 && screen) {

      fprintf(screen,"  bounding box image = (%i %i %i) to (%i %i %i)\n",

      fmt::print(screen,"  bounding box image = ({} {} {}) to ({} {} {})\n",

              _imagelo[0],_imagelo[1],_imagelo[2],

              _imagehi[0],_imagehi[1],_imagehi[2]);

      fprintf(screen,"  bounding box extra memory = %.2f MB\n",

      fmt::print(screen,"  bounding box extra memory = {:.2f} MB\n",

              (double)size_buf_all*sizeof(double)/1024/1024);

    }