- 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
- bundles frequently used 'gather/scatter' patterns more consistently.
- combineAllGather -> combineGather + broadcast
- listCombineAllGather -> listCombineGather + broadcast
- mapCombineAllGather -> mapCombineGather + broadcast
- allGatherList -> gatherList + scatterList
- reduce -> gather + broadcast (ie, allreduce)
- The allGatherList currently wraps gatherList/scatterList, but may be
replaced with a different algorithm in the future.
STYLE: PstreamCombineReduceOps.H is mostly unneeded now
