ENH: add operator[](const word&) as "find-by-name" to some classes

- affected: polyBoundary, fvBoundaryMesh, ZoneMesh, searchableSurfaces

  before:
      const label zoneI = mesh.cellZones().findZoneID(zoneName);
      const cellZone& cz = mesh.cellZones()[zoneI];
  after:
      const cellZone& cz = mesh.cellZones()[zoneName];

applications/utilities/postProcessing/graphics/PV3Readers/PV3FoamReader/vtkPV3Foam/vtkPV3FoamMeshVolume.C

@@ -328,7 +328,7 @@ vtkUnstructuredGrid* Foam::vtkPV3Foam::volumeVTKMesh
                 VTK_POLYHEDRON,
                 nodeCount,
                 uniqueNodeIds.data(),
-                faceCount,
+                nFaces,
                 faceLabels.data()
             );
 #else