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"
 

applications/utilities/postProcessing/lagrangian/particleTracks/particleTracks.C

@@ -187,8 +187,7 @@ int main(int argc, char *argv[])
         const label maxNProcs = returnReduce(maxIds.size(), maxOp<label>());
         maxIds.resize(maxNProcs, -1);
 
-        Pstream::listCombineGather(maxIds, maxEqOp<label>());
-        Pstream::broadcast(maxIds);
+        Pstream::listCombineAllGather(maxIds, maxEqOp<label>());
 
         // From ids to count
         const labelList numIds = maxIds + 1;

applications/utilities/postProcessing/miscellaneous/postChannel/channelIndexTemplates.C

@@ -40,8 +40,7 @@ Foam::Field<T> Foam::channelIndex::regionSum(const Field<T>& cellField) const
     }
 
     // Global sum
-    Pstream::listCombineGather(regionField, plusEqOp<T>());
-    Pstream::broadcast(regionField);
+    Pstream::listCombineAllGather(regionField, plusEqOp<T>());
 
     return regionField;
 }