ENH: provide MPI native bitOrOp reduce with single/multiple values

- can be used, for example, to track global states:

      // Encode as 0:empty, 1:uniform, 2:nonuniform, 3:mixed
      PackedList<2> uniformity(fields.size());

      forAll(fields, i)
      {
          uniformity.set(i, fields[i].whichUniformity());
      }

      reduce
      (
          uniformity.data(),
          uniformity.size_data(),
          bitOrOp<unsigned>()
      );