Add multi-UE support to nr_ulsim and nr_dlsim

nr_ulsim:

• Add option -x to specify the number of UEs to simulate.
  There are no hard limit to the number of UEs.
  Though all our tests above 24 UEs ended up in freezing the testbed.
• Update options -m and -r to configure each UE's MCS and allocated RB respectively.
  Arguments are comma separated lists.
• Each UE's effRate/effTP/roundStats is tracked individually

Known issues:

• One UE may have higher scrambling errors than others.
  This is because it was allocated the central frequency.
• Error: RA processes are not available for CFRA RNTI -> 717:nr_mac_add_test_ue():openair2/LAYER2/NR_MAC_gNB/config.c
  This error does not affect the simulation as far as we know.
• If SRS is enabled the test will crash for high UE numbers (~8).

nr_dlsim:

• Add option -k to specify the number of UEs to simulate, up to 16 UEs.
• Update options -e and -b to configure each UE's MCS and allocated RB respectively.
  Arguments are comma separated lists.
• Each UE's effRate/effTP/roundStats is now tracked individually
• Print more information at the end of the test

Known issues:

• Error: RA processes are not available for CFRA RNTI -> 717:nr_mac_add_test_ue():openair2/LAYER2/NR_MAC_gNB/config.c
  This error does not affect the simulation as far as we know.