- size of PUCCH has been fixed to:
    - 2 RBs for N_RB_UL = 25 (1 RB at top of resource grid, 1 RB at bottom)
    - 4 RBs for N_RB_UL = 50
    - 6 RBs for N_RB_UL = 100
  this is arbitrary and will need some rework at some point.
  This may be also wrong. PUCCH size actually depends on DL traffic
  (if the ack/nack is not done in PUSCH) and scheduling request
  configurations.
- add sched_frame %= 1024 at one needed place
- reserve RBs for retransmission
  this was not done so new transmissions were scheduled in the same
  RBs. Since the code works with the notion of 'first_rb' only, it
  was decided to skip all RBs lower than those retransmitted. This
  works but is not correct (imagine we have to retransmit RB 23, then
  all RBs < 23 will not be used for new transmission). The work to
  fix this properly is complex, a lot has to change, so let's do it
  this simple way for now.
- sort_ue_ul was not correct
  - add the function maxround_ul to use the correct 'round' (the one
    from DL was used, which is totally wrong)
  - be sure to use the correct frame/subframe to get the correct HARQ pid