[EN] OXO-NITROGENATED COMPLEX OF COBALT, CATALYTIC SYSTEM COMPRISING SAID OXO-NITROGENATED COMPLEX AND PROCESS FOR THE (CO) POLYMERIZATION OF CONJUGATED DIENES [FR] COMPLEXE DE COBALT OXO-AZOTÉ, SYSTÈME CATALYTIQUE LE COMPRENANT ET PROCÉDÉ DE (CO)POLYMÉRISATION DE DIÈNES CONJUGUÉS