v1.5.5 released

• definition of duplicated.POSIXst
• ajout des fonctions 'unit' et 'of' pour les subtimedataframe (merge ne marche pas sans ça)
• rewritting of Ops.POSIXctp
• doc improvment
• 'POSIXctpnumeric' wes not implemented but 'numericPOSIXctp' was ... now both are
• POSIXctp contructor more flexible
• adding and subtracting 'week' to POSIXct is enabled