F1: support multiple DUs per CU

• Support multiple DUs on one CU. No mobility or other cooperation between DUs is supported yet.
• The CU/gNB writes nrRRC_stats.log file and now includes DU information; UE capability information has been removed because it requires DU information, whihc might not always be available and made the CU/gNB assert (and I think it is not very useful, but we can add it again if necessary)