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-comm1/Make/files

@@ -0,0 +1,3 @@
+Test-parallel-comm1.C
+
+EXE = $(FOAM_USER_APPBIN)/Test-parallel-comm1

applications/test/parallel-comm1/Make/options

@@ -0,0 +1,2 @@
+/* EXE_INC = */
+/* EXE_LIBS = */

applications/test/parallel-comm1/Test-parallel-comm1.C

@@ -0,0 +1,185 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2013-2017 OpenFOAM Foundation
+-------------------------------------------------------------------------------
+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-communicators
+
+Description
+    Checks communication using user-defined communicators
+
+\*---------------------------------------------------------------------------*/
+
+#include "argList.H"
+#include "Time.H"
+#include "IPstream.H"
+#include "OPstream.H"
+#include "vector.H"
+#include "IOstreams.H"
+#include "PstreamReduceOps.H"
+
+using namespace Foam;
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+scalar sumReduce
+(
+    const label comm,
+    const scalar localValue
+)
+{
+    scalar sum = 0;
+    if (Pstream::parRun())
+    {
+        if (UPstream::master(comm))
+        {
+            // Add master value and all slaves
+            sum = localValue;
+
+            for (const int slave : UPstream::subProcs(comm))
+            {
+                scalar slaveValue;
+                UIPstream::read
+                (
+                    Pstream::commsTypes::blocking,
+                    slave,
+                    reinterpret_cast<char*>(&slaveValue),
+                    sizeof(scalar),
+                    UPstream::msgType(),    // tag
+                    comm                    // communicator
+                );
+
+                sum += slaveValue;
+            }
+
+            // Send back to slaves
+
+            for (const int slave : UPstream::subProcs(comm))
+            {
+                UOPstream::write
+                (
+                    UPstream::commsTypes::blocking,
+                    slave,
+                    reinterpret_cast<const char*>(&sum),
+                    sizeof(scalar),
+                    UPstream::msgType(),    // tag
+                    comm                    // communicator
+                );
+            }
+        }
+        else
+        {
+            {
+                UOPstream::write
+                (
+                    UPstream::commsTypes::blocking,
+                    UPstream::masterNo(),
+                    reinterpret_cast<const char*>(&localValue),
+                    sizeof(scalar),
+                    UPstream::msgType(),    // tag
+                    comm                    // communicator
+                );
+            }
+
+            {
+                UIPstream::read
+                (
+                    UPstream::commsTypes::blocking,
+                    UPstream::masterNo(),
+                    reinterpret_cast<char*>(&sum),
+                    sizeof(scalar),
+                    UPstream::msgType(),    // tag
+                    comm                    // communicator
+                );
+            }
+        }
+    }
+    return sum;
+}
+
+
+int main(int argc, char *argv[])
+{
+
+    #include "setRootCase.H"
+    #include "createTime.H"
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+    // Allocate a communicator
+    label n = Pstream::nProcs(UPstream::worldComm);
+
+    DynamicList<label> bottom;
+    DynamicList<label> top;
+
+    for (label i = 0; i < n/2; i++)
+    {
+        bottom.append(i);
+    }
+    for (label i = n/2; i < n; i++)
+    {
+        top.append(i);
+    }
+
+    //Pout<< "bottom:" << bottom << endl;
+    Pout<< "top             :" << top << endl;
+
+
+    scalar localValue = 111*UPstream::myProcNo(UPstream::worldComm);
+    Pout<< "localValue      :" << localValue << endl;
+
+
+    label comm = Pstream::allocateCommunicator
+    (
+        UPstream::worldComm,
+        top
+    );
+
+    Pout<< "allocated comm  :" << comm << endl;
+    Pout<< "comm myproc     :" << Pstream::myProcNo(comm)
+        << endl;
+
+
+    if (Pstream::myProcNo(comm) != -1)
+    {
+        scalar sum = returnReduce
+        (
+            localValue,
+            sumOp<scalar>(),
+            UPstream::msgType(),
+            comm
+        );
+        Pout<< "sum             :" << sum << endl;
+    }
+
+    Pstream::freeCommunicator(comm);
+
+
+    Pout<< "End\n" << endl;
+
+    return 0;
+}
+
+
+// ************************************************************************* //