1. IRISA, Campus de Beaulieu, 35042 Rennes Cedex, France (corresponding author); and CNRS.
2. Division for Metrology and Instrumentation, LCPC, 58, bd Lefebvre, 75732 Paris Cedex 15, France.
3. Centre Rennes—Bretagne Atlantique, iNRiA, Campus de Beaulieu, 35042 Rennes Cedex, France.
4. Centre de Recherche Public Henri Tudor, 29, Avenue John F. Kennedy, Kirchberg L-1855, Luxembourg.
5. Division for Metrology and Instrumentation, LCPC, Route de Pornic, BP 4129, 44341 Bouguenais Cedex, France.