ENH: add UPstream::subProcs() static method

- returns a range of `int` values that can be iterated across.
  For example,

      for (const int proci : Pstream::subProcs()) { ... }

  instead of

      for
      (
          int proci = Pstream::firstSlave();
          proci <= Pstream::lastSlave();
          ++proci
      )
      {
          ...
      }