Make USRP tune_offset configurable

UHD documentation: "A tune request instructs the implementation how to
tune the RF chain."

This commit makes this configurable, from nr-softmodem, nr-uesoftmodem,
lte-softmodem, and lte-uesoftmodem.

targets/ARCH/COMMON/common_lib.h

@@ -188,6 +188,7 @@ typedef struct {
   //! \brief Center frequency in Hz for TX.
   //! index: [0..rx_num_channels[ !!! see lte-ue.c:427 FIXME iterates over rx_num_channels
   double tx_freq[4];
+  double tune_offset;
   //! \brief memory
   //! \brief Pointer to Calibration table for RX gains
   rx_gain_calib_table_t *rx_gain_calib_table;