SC-FDMA working for 1 SNR loop (enabled using "NR_SC_FDMA" MACRO)

- Added nr_dft at UE side and nr_idft at gNB side
- NR_SC_FDMA can be found in "PHY/defs_nr_common.h"
- For 1 SNR loop, the code executes correctly
- For more loops, it gives a segmentation fault in the LDPC decoder
- The segmentation fault is caused when nr_idft is used
- The code works fine when the "NR_SC_FDMA" is disabled