- define returnReduce *after* defining all specializations for reduce
  so that the compiler does not take the generic templated reduce.

ENH: add UPstream::reduceAnd, UPstream::reduceOr

- direct wrapper of MPI_LAND, MPI_LOR intrinsics

ENH: provide special purpose returnReduce for logical operations

- returnReduceAnd(bool), returnReduceOr(bool) as a inline wrappers
  for returnReduce with andOp<bool>(), orOp<bool>() operators,
  respectively.

  These forms are more succinct and force casting of the parameter
  into a bool. Using MPI bool operations allows vendor/hardware MPI
  optimisations.

  * Test for existence on any rank:

      1.  if (returnReduceOr(list.size()) { ... }
      1b. if (returnReduceOr(!list.empty()) { ... }

      2.  if (returnReduce(bool(list.size(), orOp<bool>())) { ... }
      3.  if (returnReduce(list.size(), sumOp<label>()) != 0) { ... }
      3b. if (returnReduce(list.size(), sumOp<label>()) > 0) { ... }

  * Test for non-existence on all ranks:

      1.  if (returnReduceAnd(list.empty()) { ... }
      1b. if (!returnReduceOr(list.size()) { ... }

      2.  if (returnReduce(list.empty(), andOp<bool>())) { ... }
      3.  if (returnReduce(list.size(), sumOp<label>()) == 0) { ... }

  Notes:
      Form 1. succinct
      Form 2. may require explicit bool() for correct dispatch
      Form 3. more expensive sumOp<label> just for testing size!
      There are also some places using maxOp<label> instead of sumOp<label>