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

Merge branch 'nsa_remove_band_hardcodings' of https://gitlab.eurecom.fr/oai/openairinterface5g into nsa_remove_band_hardcodings

Maybe not the right way to do.
To be checked.

After the UE disconnects, the next connection fails
because the gNB cannot decode Msg3. The root cause
seems to be this ul_valid_re_per_slot not set to 0.
Setting it before the test fixes the problem and
does not seem to impact anything else.

Some data remains in the PHY of the gNB after UE removal by
upper layers. After some connection/deconnection/reconnection
the gNB will exit because there is no free space in PHY.

This commit adds some global variables shared by MAC and PHY.
The function mac_remove_nr_ue puts UEs to be removed in it
and the function rxtx removes what needs to be removed.

This is a hack. It will be removed at some point in time.

The real way to remove in the PHY should be to release
data after some time of inactivity in the PHY, which would
mean the UE has been removed in upper layers. Kind of
timer-based thing. There should be no communication between
PHY and MAC.

This commit 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.

The following work is included:

    - DC prep and DC overall X2 timers
    - X2 messages for UE release and DC timers' timeout

Open problems:

    - what to do when the UE sends an scg Failure? This commit removes
      the UE. And this removal may not be correctly done
    - UE removal in the gNB's PHY layer (a next commit is coming,
      but it is hackish and will need a rework)