fix signed vs. unsiged warning

  if (comm->me == 0 ) {

    fprintf(screen,"This is checkup of thylakoid %p\n", this);

    fprintf(screen,"I have %ld parts. They are:\n", parts.size());

    for( int i = 0; i < parts.size(); ++i ){

    for( int i = 0; i < (int)parts.size(); ++i ){

      fprintf(screen, "[%f, %f] x [%f, %f] x [%f, %f]\n",

              parts[i]->xlo, parts[i]->xhi,

              parts[i]->ylo, parts[i]->yhi,