ENH: improve flexiblity for flat output of items (#1929)

- change to a templated implementation instead of relying on
  the container's writeList() method.

  This inlines the generation while also adding the flexibility to
  define different delimiters (at compile time) without the
  performance penalty of passing run-time parameters.

applications/test/flatOutput1/Make/files

@@ -0,0 +1,3 @@
+Test-flatOuput1.C
+
+EXE = $(FOAM_USER_APPBIN)/Test-flatOuput1

applications/test/flatOutput1/Make/options

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

applications/test/flatOutput1/Test-flatOuput1.C

@@ -0,0 +1,103 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2020 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/>.
+
+Description
+    Simple test of FlatOutput
+
+\*---------------------------------------------------------------------------*/
+
+#include "wordList.H"
+#include "ListOps.H"
+#include "FlatOutput.H"
+#include "IOstreams.H"
+#include "macros.H"
+
+using namespace Foam;
+
+// For testing various pre-defined formatting
+#define printFlatOutput(Content, Format) \
+    STRINGIFY(Format) << ": " << flatOutput(Content, FlatOutput::Format{})
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+// Main program:
+
+int main(int argc, char *argv[])
+{
+    wordList words1
+    {
+        "ab", "cd", "ef", "gh",
+        "ij", "kl", "mn", "op",
+        "qr", "st", "uv", "wx", "yz"
+    };
+
+    {
+        Info<< nl
+            << "regular" << nl
+            << "#----------------" << nl;
+
+        Info<< nl << "operator<< " << words1 << nl;
+
+        Info<< nl << "writeList: ";
+        words1.writeList(Info) << nl;
+    }
+
+    Info<< nl
+        << "Using c++ " << int(__cplusplus) << nl;
+
+    {
+        Info<< nl
+            << "flatOutput" << nl
+            << "#----------------" << nl;
+
+        Info<< nl << "operator<< " << flatOutput(words1) << nl;
+
+        Info<< nl << "write: ";
+        flatOutput(words1).write(Info) << nl;
+
+        Info<< nl << printFlatOutput(words1, BareComma) << nl;
+        Info<< nl << printFlatOutput(words1, BareSpace) << nl;
+
+        Info<< nl << printFlatOutput(words1, BraceComma) << nl;
+        Info<< nl << printFlatOutput(words1, BraceSpace) << nl;
+
+        Info<< nl << printFlatOutput(words1, ParenComma) << nl;
+        Info<< nl << printFlatOutput(words1, ParenSpace) << nl;
+
+        Info<< nl << printFlatOutput(words1, PointyComma) << nl;
+        Info<< nl << printFlatOutput(words1, PointySpace) << nl;
+
+        Info<< nl << printFlatOutput(words1, SquareComma) << nl;
+        Info<< nl << printFlatOutput(words1, SquareSpace) << nl;
+    }
+
+
+    Info<< "\nEnd\n" << endl;
+
+    return 0;
+}
+
+
+// ************************************************************************* //