Perform 2 Step frequency adjustment in the USRP and reset frequency offset after changing the RF card frequency

It neither has any noticeable benefits nor direct relation with the intended values

small optimization in get_samples_slot_timestamp(), get_slot_from_timestamp() and get_samples_per_slot()

move allocation of UL_tti_req_ahead array from scheduling to start-up phase

  - center frequency changed
  - command line option for the ul freq scaling
  - freq offset not normalized