ENH: consolidate UPstream::commsStruct code

- make sizing of commsStruct List demand-driven as well
  for more robustness, fewer unneeded allocations.

- fix potential latent bug with allBelow/allNotBelow for proc 0
  (linear communication).

ENH: remove unused/unusable UPstream::communicator optional parameter

- had constructor option to avoid constructing the MPI backend,
  but this is not useful and inconsistent with what the reset or
  destructor expect.

STYLE: local use of UPstream::communicator

- automatically frees communicator when it leaves scope