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

applications/utilities/postProcessing/dataConversion/foamToEnsight/findCloudFields.H

@@ -104,12 +104,11 @@ if (timeDirs.size() && doLagrangian)
     {
         for (auto& cloudFields : regionCloudFields)
         {
-            Pstream::mapCombineGather
+            Pstream::mapCombineAllGather
             (
                 cloudFields,
                 HashTableOps::plusEqOp<word>()
             );
-            Pstream::broadcast(cloudFields);
         }
     }
 }

applications/utilities/postProcessing/dataConversion/foamToEnsight/foamToEnsight.C

@@ -122,7 +122,7 @@ Usage
 #include "IOobjectList.H"
 #include "IOmanip.H"
 #include "OFstream.H"
-#include "PstreamCombineReduceOps.H"
+#include "Pstream.H"
 #include "HashOps.H"
 #include "regionProperties.H"