must delete internal computes if they already exist (e.g. after a restart)

  // create computes dependent on chunks

  id_com = id + id_chunk + "_com";

  icompute = modify->find_compute(id_com);

  if (icompute >= 0) modify->delete_compute(id_com);

  auto cmd = fmt::format("{} {} com/chunk {}",id_com,group->names[igroup],id_chunk);

  modify->add_compute(cmd);

  icompute = modify->find_compute(id_com);

  ccom = (ComputeCOMChunk *) modify->compute[icompute];

  id_vcm = id + id_chunk + "_vcm";

  icompute = modify->find_compute(id_vcm);

  if (icompute >= 0) modify->delete_compute(id_vcm);

  cmd = fmt::format("{} {} vcm/chunk {}",id_vcm,group->names[igroup],id_chunk);

  modify->add_compute(cmd);

  icompute = modify->find_compute(id_vcm);

  cvcm = modify->compute[icompute];

  id_omega = id + id_chunk + "_omega";

  icompute = modify->find_compute(id_omega);

  if (icompute >= 0) modify->delete_compute(id_omega);

  cmd = fmt::format("{} {} omega/chunk {}",id_omega,group->names[igroup],id_chunk);

  modify->add_compute(cmd);

  icompute = modify->find_compute(id_omega);