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];