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

- Updated PRACH procedures and general code formatting
- gNB msg1 handling and RAR generation
- general bug fixing
- formatting
- missing headers
- RAR handling.

Signed-off-by: Raphael Defosseux <raphael.defosseux@eurecom.fr>

- merged nr-schedule_ue_spec into nr_schedule_uss_dlsch_phytest
- enabled nr_ue_send_sdu for S1 mode
- increased ta_timer to 100
- restored NR_UPLINK_SLOT to 2 (as it was in develop-nr)
- fixed logic for S1 and noS1 modes
- code cleanup and formatting

- introduced new parameter in nr_ue_dl_indication for UL time alignment
- moved TA timer logic one level up so that now is executed every slot
- set timer value to 80 slots to cope with the sl_ahead design
- introduced new TA-related members to gNB MAC instance struct

Maintenance of Uplink Time Alignment. Introduced control on TA transmission and TA update application.

- ta_timer at gNB side to control the transmission of TA to the UE. This is now happening every 20 slots.
- when the timer resets gNB is now transmitting the TA whether it is zero or not.
- introduced struct NR_UL_TIME_ALIGNMENT_t to handle configuration of time alignment related parameters.
- UE PHY RX: is now triggered through a flag when to store the TA command, the frame and slot
  when the application should occur.
- UE PHY TX: when the tx frame and slot match the time alignment configuration, it process the TA command and applies the
  TA update

Integration PHY and MAC procedures for Timing Advance updates. Compilation and run is OK. TA updates need further fixes as not working as expected.

- "nr_process_timing_advance": TA to be applied is commented out, need to be uncommented
- with introduced delay of 50 in rx_rf, TA update should be 34 in MAC CE
  but this does not occur when ta is applied at UE side according to standard
- introduced ULSCH scheduler and NR RX SDU functions
- fixes to logic in UL fill RX indication
- added crc indication in UL RX
- temporary hardcoded UE_id to 0, rnti not considered in the logic

- Initial gNB DLSCH and UE spec scheduling implentation
- MAC PDU generation (including header as per TS 38.321)
- MAC PDU processing at UE side with extracion of TA updates
- General code cleanup
- Introduced NR MAC COMMON folder, moved common files here