ENH: add more direct methods for getting meshEdge (PrimitivePatch)

- refine definition of patch boundary faces to distinguish between
  boundaryFaces() and uniqBoundaryFaces().

  * boundaryFaces() for edge to face lookup on boundary edges.
  * uniqBoundaryFaces() for accessing quantities such as face areas
    or eroding an outer layer

ENH: LabelledItem container, replaces unused 'Keyed' container

- method names in alignment with objectHit, pointIndexHit etc.
  Top-level name aligns with labelledTri.

applications/test/LabelledItem/Make/files

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

applications/test/LabelledItem/Make/options

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

applications/test/LabelledItem/Test-LabelledItem.C

@@ -0,0 +1,74 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2021 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
+    Test LabelledItem (formerly 'Keyed', but that was never used)
+
+\*---------------------------------------------------------------------------*/
+
+#include "IOstreams.H"
+#include "IOobject.H"
+#include "IFstream.H"
+#include "edge.H"
+#include "LabelledItem.H"
+#include "List.H"
+
+using namespace Foam;
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+// Main program:
+
+int main(int argc, char *argv[])
+{
+    typedef LabelledItem<edge> labelledEdge;
+
+    List<labelledEdge> edges(10);
+
+    forAll(edges, edgei)
+    {
+        auto& e = edges[edgei];
+
+        e.insert(20-edgei);
+        e.insert(edgei);
+
+        if (!(edgei % 3))
+        {
+            e.setIndex(edgei);
+        }
+    }
+
+    Info<< "edges: " << edges << nl;
+
+    Foam::sort(edges);
+
+    Info<< "sorted: " << edges << nl;
+
+    Info<< "\nEnd\n" << endl;
+
+    return 0;
+}
+
+
+// ************************************************************************* //

applications/test/primitivePatch/Test-PrimitivePatch.C

@@ -244,9 +244,10 @@ int main(int argc, char *argv[])
 
         writeFaceFaces(localPoints, localFaces, faceFaces);
 
-        const labelList bndFaceIds(pp.boundaryFaces());
+        const labelList bndFaceIds(pp.uniqBoundaryFaces());
 
-        Info<< "Have: " << bndFaceIds.size() << " boundary faces" << nl;
+        Info<< "Have: " << bndFaceIds.size()
+            << " unique boundary faces" << nl;
 
         // Can calculate by hand
         if (!pp.hasFaceCentres())