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/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())