ENH: cleaner separation of global and local world naming

- UPstream::globalComm constant always refers to MPI_COMM_WORLD but
  UPstream::worldComm could be MPI_COMM_WORLD (single world)
  or a dedicated local communicator (for multi-world).

- provide a Pstream wrapped version of MPI_COMM_SELF,
  references as UPstream::selfComm

- UPstream::isUserComm(label)
  test for additional user-defined communicators

applications/test/parallel-comm0/Test-parallel-comm0.C

@@ -0,0 +1,161 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2022 OpenCFD Ltd.
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+Application
+    Test-parallel-comm0
+
+Description
+    Very basic checks on standard communicators
+
+\*---------------------------------------------------------------------------*/
+
+#include "argList.H"
+#include "Time.H"
+#include "IPstream.H"
+#include "OPstream.H"
+#include "Pair.H"
+#include "Tuple2.H"
+#include "IOstreams.H"
+#include "PstreamReduceOps.H"
+
+using namespace Foam;
+
+
+void printInfo(const label comm)
+{
+    Info<< "comm:" << comm
+        << " nprocs:" << UPstream::nProcs(comm)
+        << " all:" << UPstream::allProcs(comm)
+        << " sub:" << UPstream::subProcs(comm) << nl;
+
+
+    if (UPstream::selfComm == comm)
+    {
+        Pout<< "self all:" << UPstream::allProcs(comm)
+            << " sub:" << UPstream::subProcs(comm) << nl;
+    }
+}
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+int main(int argc, char *argv[])
+{
+    argList::noBanner();
+    argList::noCheckProcessorDirectories();
+    argList::addBoolOption("verbose", "Set debug level");
+
+    // Capture manually. We need values before proper startup
+    int nVerbose = 0;
+    for (int argi = 1; argi < argc; ++argi)
+    {
+        if (strcmp(argv[argi], "-verbose") == 0)
+        {
+            ++nVerbose;
+        }
+    }
+
+    UPstream::debug = nVerbose;
+
+
+    #include "setRootCase.H"
+
+    Info<< nl
+        << "nProcs = " << UPstream::nProcs()
+        << " with " << UPstream::nComms() << " predefined comm(s)" << nl;
+
+    Info<< "worldComm : ";
+    printInfo(UPstream::worldComm);
+
+    Info<< "selfComm  : ";
+    printInfo(UPstream::selfComm);
+
+    Info<< nl;
+
+    // Reductions (using MPI intrinsics)
+    {
+        label val = Pstream::myProcNo(UPstream::worldComm);
+
+        label worldVal = returnReduce
+        (
+            val,
+            sumOp<label>(),
+            Pstream::msgType(),
+            UPstream::worldComm
+        );
+
+        label selfVal = returnReduce
+        (
+            val,
+            sumOp<label>(),
+            Pstream::msgType(),
+            UPstream::selfComm
+        );
+
+        Pout<< "value " << val
+            << " (world) reduced " << worldVal
+            << " (self) reduced " << selfVal << nl;
+    }
+
+    // Reductions (not using MPI intrinsics)
+    {
+        Pair<label> val
+        (
+            Pstream::myProcNo(UPstream::worldComm),
+            Pstream::myProcNo(UPstream::worldComm)
+        );
+
+        Pair<label> worldVal = val;
+
+        Pstream::combineReduce
+        (
+            worldVal,
+            minFirstEqOp<label>(),
+            Pstream::msgType(),
+            UPstream::worldComm
+        );
+
+        Pair<label> selfVal = val;
+
+        Pstream::combineReduce
+        (
+            worldVal,
+            minFirstEqOp<label>(),
+            Pstream::msgType(),
+            UPstream::selfComm
+        );
+
+        Pout<< "value " << val
+            << " (world) reduced " << worldVal
+            << " (self) reduced " << selfVal << nl;
+    }
+
+    Pout<< "\nEnd\n" << endl;
+
+    return 0;
+}
+
+
+// ************************************************************************* //