The name NR_UE_list_t is misleading: beyond being a simple "list", this
structure carries all/most UE-related MAC information. If at all, it is
a container for multiple lists. The next commit introduces a
NR_UE_list_t structure that organizes UEs in lists such that it is
easier to go through the UEs.

when in phy test, only schedule UE to be in slot 8

nr_configure_pdcch() allocates the CCE, but this is not wanted, e.g., in
multi-user scheduling, since it might not be possible to allocate the
CCEs for these UEs. This commit makes the following changes:

- remove CCE allocation in nr_configure_pdcch()
- remove coreset search in nr_configure_pdcch()
- add functions get_coreset() and get_searchspace() to retrieve coresets
  and search spaces easily
- in nr_fill_nfapi_dl_pdu(), add extra parameters for aggregation level
  and CCE indices and the searchspace and coreset of a UE, which might
  be different for each UE
- allocate CCEs manually in phytest, RA, and DLSCH schedulers

MR 896 : Nsa remove band hardcodings  
Removes hardcodings of EUTRA and NR bands in UE_Capability_Enquiry and NR_UE Capability_Enquiry messages, providing them from the eNB config file and gNB over X2 respectively. Moreover, the SSB Absolute frequency of the gNB is now provided by the eNB configuration file (instead of being hardcoded at rrc_eNB.c)  

MR903 : pusch ta update  
Update of PUSCH Timing advance  

MR 900 : improve x2/rrc for ENDC  
This MR introduces several enhancements in X2AP and RRC to deal with ENDC. 
The main goal is to deal with UE's on/off and with various errors.
Includes :
- DC prep and DC overall X2 timers
- X2 messages for UE release and DC timers' timeout