1. Adding a "SL" frequency for Sidelink mode. The
   importance of this variable is to ensure that
   when we are in mode 1 (UE+UE+gNB), that the slot
   configuration and frequencies do not overlap. This
   SL frequency is important for rotation and allows
   us to utilize the OFDM modulation and demodulation
   functions that already exist in OAI.
2. Adding sl_mode to the command line for proper UE
   deployment when in SL mode. sl_mode can be set to 0,
   1, or 2. sl_mode 0 indicates no-Sidelink at all,
   mode 1 is gNB, UE, and UE (3GPP defines mode numbers).
   sl_mode 2 indicated UE to UE communications with no
   gNB at all.
3. New functions for calculating the carrier frequency
   for SL mode.