ENH: bundle Pstream:: AllGather methods

- 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

src/parallel/reconstruct/faReconstruct/faMeshReconstructor.C

@@ -30,7 +30,6 @@ License
 #include "globalMeshData.H"
 #include "edgeHashes.H"
 #include "Time.H"
-#include "PstreamCombineReduceOps.H"
 
 // * * * * * * * * * * * * * * Static Data Members * * * * * * * * * * * * * //
 
@@ -386,7 +385,7 @@ void Foam::faMeshReconstructor::calcAddressing
 
 
         // Collect from all processors
-        combineReduce
+        Pstream::combineAllGather
         (
             patchEdgeLabels,
             ListOps::appendEqOp<label>()