ENH: provide MPI native minOp, maxOp reduce multiple values

- consistent with sumOp

ENH: globalIndex with gatherNonLocal tag, and use leading dispatch tags

- useful for gather/write where the master data can be written
 separately.  Leading vs trailing dispatch tags for more similarity to
 other C++ conventions.

applications/utilities/mesh/manipulation/checkMesh/checkTools.C

@@ -428,7 +428,7 @@ void Foam::mergeAndWrite
         // Renumber local ids -> global ids
         globalIndex(mesh.nPoints()).inplaceToGlobal(mergedIDs);
 
-        globalIndex gatherer(mergedIDs.size(), globalIndex::gatherOnly{});
+        globalIndex gatherer(globalIndex::gatherOnly{}, mergedIDs.size());
         gatherer.gatherInplace(mergedIDs);
         gatherer.gatherInplace(mergedPts);
     }