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/parallelProcessing/decomposePar/domainDecompositionDistribute.C

@@ -55,7 +55,7 @@ void Foam::domainDecomposition::distributeCells()
 
         forAll(pNames, i)
         {
-            label patchI = patches.findPatchID(pNames[i]);
+            const label patchI = patches.findPatchID(pNames[i]);
 
             if (patchI == -1)
             {