ENH: add UPstream::parRun(const bool)

- slightly neater code when disabling/restoring parallel state

  New
      const bool oldParRun = Pstream::parRun(false);
      ...
      Pstream::parRun(oldParRun);

  Old
      const bool oldParRun = Pstream::parRun();
      Pstream::parRun() = false;
      ...
      Pstream::parRun() = oldParRun;